From: Ben Pfaff <blp@nicira.com>
Date: Wed, 28 Dec 2011 20:42:14 +0000 (-0800)
Subject: meta-flow: New "subfield" data structure.
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=816fd533f85923c03cf8d9d6450bd9a0845d5160;p=openvswitch

meta-flow: New "subfield" data structure.

Until now, parts of a field have been dealt with in a fairly ad-hoc way.
struct mf_subfield and the supporting functions added by this commit make
their use more systematic.

Signed-off-by: Ben Pfaff <blp@nicira.com>
---

diff --git a/lib/autopath.c b/lib/autopath.c
index 321b106d..b8ba3c17 100644
--- a/lib/autopath.c
+++ b/lib/autopath.c
@@ -22,6 +22,7 @@
 #include <stdlib.h>
 
 #include "flow.h"
+#include "meta-flow.h"
 #include "nx-match.h"
 #include "ofp-errors.h"
 #include "ofp-util.h"
@@ -36,23 +37,26 @@ void
 autopath_execute(const struct nx_action_autopath *ap, struct flow *flow,
                  uint16_t ofp_port)
 {
-    nxm_reg_load(ap->dst, ap->ofs_nbits, ofp_port, flow);
+    struct mf_subfield dst;
+
+    nxm_decode(&dst, ap->dst, ap->ofs_nbits);
+    mf_set_subfield_value(&dst, ofp_port, flow);
 }
 
 void
 autopath_parse(struct nx_action_autopath *ap, const char *s_)
 {
     char *s;
-    uint32_t reg;
-    int id_int, ofs, n_bits;
-    char *id_str, *dst, *save_ptr;
+    char *id_str, *dst_s, *save_ptr;
+    struct mf_subfield dst;
+    int id_int;
 
     s = xstrdup(s_);
     save_ptr = NULL;
     id_str = strtok_r(s, ", ", &save_ptr);
-    dst = strtok_r(NULL, ", ", &save_ptr);
+    dst_s = strtok_r(NULL, ", ", &save_ptr);
 
-    if (!dst) {
+    if (!dst_s) {
         ovs_fatal(0, "%s: not enough arguments to autopath action", s_);
     }
 
@@ -62,16 +66,17 @@ autopath_parse(struct nx_action_autopath *ap, const char *s_)
                   "1 to %"PRIu32, s_, id_int, UINT32_MAX);
     }
 
-    nxm_parse_field_bits(dst, &reg, &ofs, &n_bits);
-    if (n_bits < 16) {
+    mf_parse_subfield(&dst, dst_s);
+    if (dst.n_bits < 16) {
         ovs_fatal(0, "%s: %d-bit destination field has %u possible values, "
-                  "less than required 65536", s_, n_bits, 1u << n_bits);
+                  "less than required 65536",
+                  s_, dst.n_bits, 1u << dst.n_bits);
     }
 
     ofputil_init_NXAST_AUTOPATH(ap);
     ap->id = htonl(id_int);
-    ap->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
-    ap->dst = htonl(reg);
+    ap->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
+    ap->dst = htonl(dst.field->nxm_header);
 
     free(s);
 }
@@ -79,14 +84,14 @@ autopath_parse(struct nx_action_autopath *ap, const char *s_)
 enum ofperr
 autopath_check(const struct nx_action_autopath *ap, const struct flow *flow)
 {
-    int n_bits = nxm_decode_n_bits(ap->ofs_nbits);
-    int ofs = nxm_decode_ofs(ap->ofs_nbits);
+    struct mf_subfield dst;
 
-    if (n_bits < 16) {
+    nxm_decode(&dst, ap->dst, ap->ofs_nbits);
+    if (dst.n_bits < 16) {
         VLOG_WARN("at least 16 bit destination is required for autopath "
                   "action.");
         return OFPERR_OFPBAC_BAD_ARGUMENT;
     }
 
-    return nxm_dst_check(ap->dst, ofs, n_bits, flow);
+    return mf_check_dst(&dst, flow);
 }
diff --git a/lib/bundle.c b/lib/bundle.c
index 8e98fb57..733d79a9 100644
--- a/lib/bundle.c
+++ b/lib/bundle.c
@@ -22,6 +22,7 @@
 
 #include "dynamic-string.h"
 #include "multipath.h"
+#include "meta-flow.h"
 #include "nx-match.h"
 #include "ofpbuf.h"
 #include "ofp-errors.h"
@@ -94,8 +95,11 @@ bundle_execute_load(const struct nx_action_bundle *nab, struct flow *flow,
                     bool (*slave_enabled)(uint16_t ofp_port, void *aux),
                     void *aux)
 {
-    nxm_reg_load(nab->dst, nab->ofs_nbits,
-                 bundle_execute(nab, flow, slave_enabled, aux), flow);
+    struct mf_subfield dst;
+
+    nxm_decode(&dst, nab->dst, nab->ofs_nbits);
+    mf_set_subfield_value(&dst, bundle_execute(nab, flow, slave_enabled, aux),
+                          flow);
 }
 
 /* Checks that 'nab' specifies a bundle action which is supported by this
@@ -146,15 +150,15 @@ bundle_check(const struct nx_action_bundle *nab, int max_ports,
     }
 
     if (subtype == NXAST_BUNDLE_LOAD) {
-        int ofs = nxm_decode_ofs(nab->ofs_nbits);
-        int n_bits = nxm_decode_n_bits(nab->ofs_nbits);
+        struct mf_subfield dst;
 
-        if (n_bits < 16) {
+        nxm_decode(&dst, nab->dst, nab->ofs_nbits);
+        if (dst.n_bits < 16) {
             VLOG_WARN_RL(&rl, "bundle_load action requires at least 16 bit "
                          "destination.");
             error = OFPERR_OFPBAC_BAD_ARGUMENT;
         } else if (!error) {
-            error = nxm_dst_check(nab->dst, ofs, n_bits, flow);
+            error = mf_check_dst(&dst, flow);
         }
     }
 
@@ -192,7 +196,7 @@ bundle_check(const struct nx_action_bundle *nab, int max_ports,
 static void
 bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
                const char *fields, const char *basis, const char *algorithm,
-               const char *slave_type, const char *dst,
+               const char *slave_type, const char *dst_s,
                const char *slave_delim)
 {
     enum ofputil_action_code code;
@@ -208,7 +212,7 @@ bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
                    s, slave_delim);
     }
 
-    code = dst ? OFPUTIL_NXAST_BUNDLE_LOAD : OFPUTIL_NXAST_BUNDLE;
+    code = dst_s ? OFPUTIL_NXAST_BUNDLE_LOAD : OFPUTIL_NXAST_BUNDLE;
     b->l2 = ofputil_put_action(code, b);
 
     n_slaves = 0;
@@ -259,14 +263,12 @@ bundle_parse__(struct ofpbuf *b, const char *s, char **save_ptr,
         ovs_fatal(0, "%s: unknown slave_type `%s'", s, slave_type);
     }
 
-    if (dst) {
-        uint32_t reg;
-        int ofs, n_bits;
-
-        nxm_parse_field_bits(dst, &reg, &ofs, &n_bits);
+    if (dst_s) {
+        struct mf_subfield dst;
 
-        nab->dst = htonl(reg);
-        nab->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
+        mf_parse_subfield(&dst, dst_s);
+        nab->dst = htonl(dst.field->nxm_header);
+        nab->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
     }
 
     b->l2 = NULL;
@@ -359,9 +361,10 @@ bundle_format(const struct nx_action_bundle *nab, struct ds *s)
                   ntohs(nab->basis), algorithm, slave_type);
 
     if (nab->subtype == htons(NXAST_BUNDLE_LOAD)) {
-        nxm_format_field_bits(s, ntohl(nab->dst),
-                              nxm_decode_ofs(nab->ofs_nbits),
-                              nxm_decode_n_bits(nab->ofs_nbits));
+        struct mf_subfield dst;
+
+        nxm_decode(&dst, nab->dst, nab->ofs_nbits);
+        mf_format_subfield(&dst, s);
         ds_put_cstr(s, ",");
     }
 
diff --git a/lib/learn.c b/lib/learn.c
index 241f3d1f..16764621 100644
--- a/lib/learn.c
+++ b/lib/learn.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 Nicira Networks.
+ * Copyright (c) 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -59,6 +59,14 @@ get_bits(int n_bits, const void **p)
     return value;
 }
 
+static void
+get_subfield(int n_bits, const void **p, struct mf_subfield *sf)
+{
+    sf->field = mf_from_nxm_header(ntohl(get_be32(p)));
+    sf->ofs = ntohs(get_be16(p));
+    sf->n_bits = n_bits;
+}
+
 static unsigned int
 learn_min_len(uint16_t header)
 {
@@ -143,10 +151,10 @@ learn_check(const struct nx_action_learn *learn, const struct flow *flow)
 
         /* Check the source. */
         if (src_type == NX_LEARN_SRC_FIELD) {
-            ovs_be32 src_field = get_be32(&p);
-            int src_ofs = ntohs(get_be16(&p));
+            struct mf_subfield src;
 
-            error = nxm_src_check(src_field, src_ofs, n_bits, flow);
+            get_subfield(n_bits, &p, &src);
+            error = mf_check_src(&src, flow);
             if (error) {
                 return error;
             }
@@ -157,20 +165,19 @@ learn_check(const struct nx_action_learn *learn, const struct flow *flow)
 
         /* Check the destination. */
         if (dst_type == NX_LEARN_DST_MATCH || dst_type == NX_LEARN_DST_LOAD) {
-            ovs_be32 dst_field = get_be32(&p);
-            int dst_ofs = ntohs(get_be16(&p));
+            struct mf_subfield dst;
 
+            get_subfield(n_bits, &p, &dst);
             error = (dst_type == NX_LEARN_DST_LOAD
-                     ? nxm_dst_check(dst_field, dst_ofs, n_bits, &rule.flow)
-                     : nxm_src_check(dst_field, dst_ofs, n_bits, &rule.flow));
+                     ? mf_check_dst(&dst, &rule.flow)
+                     : mf_check_src(&dst, &rule.flow));
             if (error) {
                 return error;
             }
 
             if (dst_type == NX_LEARN_DST_MATCH
                 && src_type == NX_LEARN_SRC_IMMEDIATE) {
-                mf_set_subfield(mf_from_nxm_header(ntohl(dst_field)), value,
-                                dst_ofs, n_bits, &rule);
+                mf_set_subfield(&dst, value, &rule);
             }
         }
     }
@@ -210,38 +217,32 @@ learn_execute(const struct nx_action_learn *learn, const struct flow *flow,
         uint64_t value;
 
         struct nx_action_reg_load *load;
-        ovs_be32 dst_field;
-        int dst_ofs;
+        struct mf_subfield dst;
 
         if (!header) {
             break;
         }
 
         if (src_type == NX_LEARN_SRC_FIELD) {
-            ovs_be32 src_field = get_be32(&p);
-            int src_ofs = ntohs(get_be16(&p));
+            struct mf_subfield src;
 
-            value = nxm_read_field_bits(src_field,
-                                        nxm_encode_ofs_nbits(src_ofs, n_bits),
-                                        flow);
+            get_subfield(n_bits, &p, &src);
+            value = mf_get_subfield(&src, flow);
         } else {
             value = get_bits(n_bits, &p);
         }
 
         switch (dst_type) {
         case NX_LEARN_DST_MATCH:
-            dst_field = get_be32(&p);
-            dst_ofs = ntohs(get_be16(&p));
-            mf_set_subfield(mf_from_nxm_header(ntohl(dst_field)), value,
-                            dst_ofs, n_bits, &fm->cr);
+            get_subfield(n_bits, &p, &dst);
+            mf_set_subfield(&dst, value, &fm->cr);
             break;
 
         case NX_LEARN_DST_LOAD:
-            dst_field = get_be32(&p);
-            dst_ofs = ntohs(get_be16(&p));
+            get_subfield(n_bits, &p, &dst);
             load = ofputil_put_NXAST_REG_LOAD(&actions);
-            load->ofs_nbits = nxm_encode_ofs_nbits(dst_ofs, n_bits);
-            load->dst = dst_field;
+            load->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
+            load->dst = htonl(dst.field->nxm_header);
             load->value = htonll(value);
             break;
 
@@ -283,19 +284,18 @@ struct learn_spec {
     int n_bits;
 
     int src_type;
-    const struct mf_field *src;
-    int src_ofs;
+    struct mf_subfield src;
     uint8_t src_imm[sizeof(union mf_value)];
 
     int dst_type;
-    const struct mf_field *dst;
-    int dst_ofs;
+    struct mf_subfield dst;
 };
 
 static void
 learn_parse_spec(const char *orig, char *name, char *value,
                  struct learn_spec *spec)
 {
+    memset(spec, 0, sizeof *spec);
     if (mf_from_name(name)) {
         const struct mf_field *dst = mf_from_name(name);
         union mf_value imm;
@@ -308,49 +308,36 @@ learn_parse_spec(const char *orig, char *name, char *value,
 
         spec->n_bits = dst->n_bits;
         spec->src_type = NX_LEARN_SRC_IMMEDIATE;
-        spec->src = NULL;
-        spec->src_ofs = 0;
         memcpy(spec->src_imm, &imm, dst->n_bytes);
         spec->dst_type = NX_LEARN_DST_MATCH;
-        spec->dst = dst;
-        spec->dst_ofs = 0;
+        spec->dst.field = dst;
+        spec->dst.ofs = 0;
+        spec->dst.n_bits = dst->n_bits;
     } else if (strchr(name, '[')) {
-        uint32_t src_header, dst_header;
-        int src_ofs, dst_ofs;
-        int n_bits;
-
         /* Parse destination and check prerequisites. */
-        if (nxm_parse_field_bits(name, &dst_header, &dst_ofs,
-                                 &n_bits)[0] != '\0') {
+        if (mf_parse_subfield(&spec->dst, name)[0] != '\0') {
             ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
                       orig, name);
         }
 
         /* Parse source and check prerequisites. */
         if (value[0] != '\0') {
-            int src_nbits;
-
-            if (nxm_parse_field_bits(value, &src_header, &src_ofs,
-                                     &src_nbits)[0] != '\0') {
+            if (mf_parse_subfield(&spec->src, value)[0] != '\0') {
                 ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
                           orig, value);
             }
-            if (src_nbits != n_bits) {
-                ovs_fatal(0, "%s: bit widths of %s (%d) and %s (%d) differ",
-                          orig, name, dst_header, value, dst_header);
+            if (spec->src.n_bits != spec->dst.n_bits) {
+                ovs_fatal(0, "%s: bit widths of %s (%u) and %s (%u) differ",
+                          orig, name, spec->src.n_bits, value,
+                          spec->dst.n_bits);
             }
         } else {
-            src_header = dst_header;
-            src_ofs = dst_ofs;
+            spec->src = spec->dst;
         }
 
-        spec->n_bits = n_bits;
+        spec->n_bits = spec->src.n_bits;
         spec->src_type = NX_LEARN_SRC_FIELD;
-        spec->src = mf_from_nxm_header(src_header);
-        spec->src_ofs = src_ofs;
         spec->dst_type = NX_LEARN_DST_MATCH;
-        spec->dst = mf_from_nxm_header(dst_header);
-        spec->dst_ofs = 0;
     } else if (!strcmp(name, "load")) {
         if (value[strcspn(value, "[-")] == '-') {
             struct nx_action_reg_load load;
@@ -365,14 +352,11 @@ learn_parse_spec(const char *orig, char *name, char *value,
 
             spec->n_bits = nbits;
             spec->src_type = NX_LEARN_SRC_IMMEDIATE;
-            spec->src = NULL;
-            spec->src_ofs = 0;
             for (i = 0; i < imm_bytes; i++) {
                 spec->src_imm[i] = imm >> ((imm_bytes - i - 1) * 8);
             }
             spec->dst_type = NX_LEARN_DST_LOAD;
-            spec->dst = mf_from_nxm_header(ntohl(load.dst));
-            spec->dst_ofs = nxm_decode_ofs(load.ofs_nbits);
+            nxm_decode(&spec->dst, load.dst, load.ofs_nbits);
         } else {
             struct nx_action_reg_move move;
 
@@ -380,28 +364,21 @@ learn_parse_spec(const char *orig, char *name, char *value,
 
             spec->n_bits = ntohs(move.n_bits);
             spec->src_type = NX_LEARN_SRC_FIELD;
-            spec->src = mf_from_nxm_header(ntohl(move.src));
-            spec->src_ofs = ntohs(move.src_ofs);
+            nxm_decode_discrete(&spec->src,
+                                move.src, move.src_ofs, move.n_bits);
             spec->dst_type = NX_LEARN_DST_LOAD;
-            spec->dst = mf_from_nxm_header(ntohl(move.dst));
-            spec->dst_ofs = ntohs(move.dst_ofs);
+            nxm_decode_discrete(&spec->dst,
+                                move.dst, move.dst_ofs, move.n_bits);
         }
     } else if (!strcmp(name, "output")) {
-        uint32_t header;
-        int ofs, n_bits;
-
-        if (nxm_parse_field_bits(value, &header, &ofs, &n_bits)[0] != '\0') {
+        if (mf_parse_subfield(&spec->src, value)[0] != '\0') {
             ovs_fatal(0, "%s: syntax error after NXM field name `%s'",
                       orig, name);
         }
 
-        spec->n_bits = n_bits;
+        spec->n_bits = spec->src.n_bits;
         spec->src_type = NX_LEARN_SRC_FIELD;
-        spec->src = mf_from_nxm_header(header);
-        spec->src_ofs = ofs;
         spec->dst_type = NX_LEARN_DST_OUTPUT;
-        spec->dst = NULL;
-        spec->dst_ofs = 0;
     } else {
         ovs_fatal(0, "%s: unknown keyword %s", orig, name);
     }
@@ -452,29 +429,29 @@ learn_parse(struct ofpbuf *b, char *arg, const struct flow *flow)
 
             /* Check prerequisites. */
             if (spec.src_type == NX_LEARN_SRC_FIELD
-                && !mf_are_prereqs_ok(spec.src, flow)) {
+                && !mf_are_prereqs_ok(spec.src.field, flow)) {
                 ovs_fatal(0, "%s: cannot specify source field %s because "
                           "prerequisites are not satisfied",
-                          orig, spec.src->name);
+                          orig, spec.src.field->name);
             }
             if ((spec.dst_type == NX_LEARN_DST_MATCH
                  || spec.dst_type == NX_LEARN_DST_LOAD)
-                && !mf_are_prereqs_ok(spec.dst, &rule.flow)) {
+                && !mf_are_prereqs_ok(spec.dst.field, &rule.flow)) {
                 ovs_fatal(0, "%s: cannot specify destination field %s because "
                           "prerequisites are not satisfied",
-                          orig, spec.dst->name);
+                          orig, spec.dst.field->name);
             }
 
             /* Update 'rule' to allow for satisfying destination
              * prerequisites. */
             if (spec.src_type == NX_LEARN_SRC_IMMEDIATE
                 && spec.dst_type == NX_LEARN_DST_MATCH
-                && spec.dst_ofs == 0
-                && spec.n_bits == spec.dst->n_bytes * 8) {
+                && spec.dst.ofs == 0
+                && spec.n_bits == spec.dst.field->n_bytes * 8) {
                 union mf_value imm;
 
-                memcpy(&imm, spec.src_imm, spec.dst->n_bytes);
-                mf_set_value(spec.dst, &imm, &rule);
+                memcpy(&imm, spec.src_imm, spec.dst.field->n_bytes);
+                mf_set_value(spec.dst.field, &imm, &rule);
             }
 
             /* Output the flow_mod_spec. */
@@ -486,13 +463,13 @@ learn_parse(struct ofpbuf *b, char *arg, const struct flow *flow)
                 }
                 ofpbuf_put(b, spec.src_imm, n_bytes);
             } else {
-                put_u32(b, spec.src->nxm_header);
-                put_u16(b, spec.src_ofs);
+                put_u32(b, spec.src.field->nxm_header);
+                put_u16(b, spec.src.ofs);
             }
             if (spec.dst_type == NX_LEARN_DST_MATCH ||
                 spec.dst_type == NX_LEARN_DST_LOAD) {
-                put_u32(b, spec.dst->nxm_header);
-                put_u16(b, spec.dst_ofs);
+                put_u32(b, spec.dst.field->nxm_header);
+                put_u16(b, spec.dst.ofs);
             } else {
                 assert(spec.dst_type == NX_LEARN_DST_OUTPUT);
             }
@@ -555,15 +532,12 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
         int n_bits = header & NX_LEARN_N_BITS_MASK;
 
         int src_type = header & NX_LEARN_SRC_MASK;
-        uint32_t src_header;
-        int src_ofs;
+        struct mf_subfield src;
         const uint8_t *src_value;
         int src_value_bytes;
 
         int dst_type = header & NX_LEARN_DST_MASK;
-        uint32_t dst_header;
-        int dst_ofs;
-        const struct mf_field *dst_field;
+        struct mf_subfield dst;
 
         enum ofperr error;
         int i;
@@ -589,13 +563,13 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
 
         /* Get the source. */
         if (src_type == NX_LEARN_SRC_FIELD) {
-            src_header = ntohl(get_be32(&p));
-            src_ofs = ntohs(get_be16(&p));
+            get_subfield(n_bits, &p, &src);
             src_value_bytes = 0;
             src_value = NULL;
         } else {
-            src_header = 0;
-            src_ofs = 0;
+            src.field = NULL;
+            src.ofs = 0;
+            src.n_bits = 0;
             src_value_bytes = 2 * DIV_ROUND_UP(n_bits, 16);
             src_value = p;
             p = (const void *) ((const uint8_t *) p + src_value_bytes);
@@ -603,41 +577,39 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
 
         /* Get the destination. */
         if (dst_type == NX_LEARN_DST_MATCH || dst_type == NX_LEARN_DST_LOAD) {
-            dst_header = ntohl(get_be32(&p));
-            dst_field = mf_from_nxm_header(dst_header);
-            dst_ofs = ntohs(get_be16(&p));
+            get_subfield(n_bits, &p, &dst);
         } else {
-            dst_header = 0;
-            dst_field = NULL;
-            dst_ofs = 0;
+            dst.field = NULL;
+            dst.ofs = 0;
+            dst.n_bits = 0;
         }
 
         ds_put_char(s, ',');
 
         switch (src_type | dst_type) {
         case NX_LEARN_SRC_IMMEDIATE | NX_LEARN_DST_MATCH:
-            if (dst_field && dst_ofs == 0 && n_bits == dst_field->n_bits) {
+            if (dst.field && dst.ofs == 0 && n_bits == dst.field->n_bits) {
                 union mf_value value;
                 uint8_t *bytes = (uint8_t *) &value;
 
-                if (src_value_bytes > dst_field->n_bytes) {
+                if (src_value_bytes > dst.field->n_bytes) {
                     /* The destination field is an odd number of bytes, which
                      * got rounded up to a multiple of 2 to be put into the
                      * learning action.  Skip over the leading byte, which
                      * should be zero anyway.  Otherwise the memcpy() below
                      * will overrun the start of 'value'. */
-                    int diff = src_value_bytes - dst_field->n_bytes;
+                    int diff = src_value_bytes - dst.field->n_bytes;
                     src_value += diff;
                     src_value_bytes -= diff;
                 }
 
                 memset(&value, 0, sizeof value);
-                memcpy(&bytes[dst_field->n_bytes - src_value_bytes],
+                memcpy(&bytes[dst.field->n_bytes - src_value_bytes],
                        src_value, src_value_bytes);
-                ds_put_format(s, "%s=", dst_field->name);
-                mf_format(dst_field, &value, NULL, s);
+                ds_put_format(s, "%s=", dst.field->name);
+                mf_format(dst.field, &value, NULL, s);
             } else {
-                nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
+                mf_format_subfield(&dst, s);
                 ds_put_cstr(s, "=0x");
                 for (i = 0; i < src_value_bytes; i++) {
                     ds_put_format(s, "%02"PRIx8, src_value[i]);
@@ -646,10 +618,10 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
             break;
 
         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_MATCH:
-            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
-            if (src_header != dst_header || src_ofs != dst_ofs) {
+            mf_format_subfield(&dst, s);
+            if (src.field != dst.field || src.ofs != dst.ofs) {
                 ds_put_char(s, '=');
-                nxm_format_field_bits(s, src_header, src_ofs, n_bits);
+                mf_format_subfield(&src, s);
             }
             break;
 
@@ -659,19 +631,19 @@ learn_format(const struct nx_action_learn *learn, struct ds *s)
                 ds_put_format(s, "%02"PRIx8, src_value[i]);
             }
             ds_put_cstr(s, "->");
-            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
+            mf_format_subfield(&dst, s);
             break;
 
         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_LOAD:
             ds_put_cstr(s, "load:");
-            nxm_format_field_bits(s, src_header, src_ofs, n_bits);
+            mf_format_subfield(&src, s);
             ds_put_cstr(s, "->");
-            nxm_format_field_bits(s, dst_header, dst_ofs, n_bits);
+            mf_format_subfield(&dst, s);
             break;
 
         case NX_LEARN_SRC_FIELD | NX_LEARN_DST_OUTPUT:
             ds_put_cstr(s, "output:");
-            nxm_format_field_bits(s, src_header, src_ofs, n_bits);
+            mf_format_subfield(&src, s);
             break;
         }
     }
diff --git a/lib/meta-flow.c b/lib/meta-flow.c
index 41cbf494..a0e2aa82 100644
--- a/lib/meta-flow.c
+++ b/lib/meta-flow.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 Nicira Networks.
+ * Copyright (c) 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,12 +26,16 @@
 
 #include "classifier.h"
 #include "dynamic-string.h"
+#include "ofp-errors.h"
 #include "ofp-util.h"
 #include "packets.h"
 #include "random.h"
 #include "shash.h"
 #include "socket-util.h"
 #include "unaligned.h"
+#include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(meta_flow);
 
 #define MF_FIELD_SIZES(MEMBER)                  \
     sizeof ((union mf_value *)0)->MEMBER,       \
@@ -394,6 +398,10 @@ struct nxm_field {
 
 static struct hmap all_nxm_fields = HMAP_INITIALIZER(&all_nxm_fields);
 
+/* Rate limit for parse errors.  These always indicate a bug in an OpenFlow
+ * controller and so there's not much point in showing a lot of them. */
+static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 5);
+
 /* Returns the field with the given 'id'. */
 const struct mf_field *
 mf_from_id(enum mf_field_id id)
@@ -1621,77 +1629,53 @@ mf_set(const struct mf_field *mf,
     }
 }
 
-/* Makes a subfield starting at bit offset 'ofs' and continuing for 'n_bits' in
- * 'rule''s field 'mf' exactly match the 'n_bits' least-significant bits of
- * 'x'.
- *
- * Example: suppose that 'mf' is originally the following 2-byte field in
- * 'rule':
- *
- *     value == 0xe00a == 2#1110000000001010
- *      mask == 0xfc3f == 2#1111110000111111
- *
- * The call mf_set_subfield(mf, 0x55, 8, 7, rule) would have the following
- * effect (note that 0x55 is 2#1010101):
- *
- *     value == 0xd50a == 2#1101010100001010
- *      mask == 0xff3f == 2#1111111100111111
- *
- * The caller is responsible for ensuring that the result will be a valid
- * wildcard pattern for 'mf'.  The caller is responsible for ensuring that
- * 'rule' meets 'mf''s prerequisites. */
-void
-mf_set_subfield(const struct mf_field *mf, uint64_t x, unsigned int ofs,
-                unsigned int n_bits, struct cls_rule *rule)
+static enum ofperr
+mf_check__(const struct mf_subfield *sf, const struct flow *flow,
+           const char *type)
 {
-    if (ofs == 0 && mf->n_bytes * 8 == n_bits) {
-        union mf_value value;
-        int i;
-
-        for (i = mf->n_bytes - 1; i >= 0; i--) {
-            ((uint8_t *) &value)[i] = x;
-            x >>= 8;
-        }
-        mf_set_value(mf, &value, rule);
+    if (!sf->field) {
+        VLOG_WARN_RL(&rl, "unknown %s field", type);
+    } else if (!sf->n_bits) {
+        VLOG_WARN_RL(&rl, "zero bit %s field %s", type, sf->field->name);
+    } else if (sf->ofs >= sf->field->n_bits) {
+        VLOG_WARN_RL(&rl, "bit offset %d exceeds %d-bit width of %s field %s",
+                     sf->ofs, sf->field->n_bits, type, sf->field->name);
+    } else if (sf->ofs + sf->n_bits > sf->field->n_bits) {
+        VLOG_WARN_RL(&rl, "bit offset %d and width %d exceeds %d-bit width "
+                     "of %s field %s", sf->ofs, sf->n_bits,
+                     sf->field->n_bits, type, sf->field->name);
+    } else if (flow && !mf_are_prereqs_ok(sf->field, flow)) {
+        VLOG_WARN_RL(&rl, "%s field %s lacks correct prerequisites",
+                     type, sf->field->name);
     } else {
-        union mf_value value, mask;
-        uint8_t *vp, *mp;
-        unsigned int byte_ofs;
-
-        mf_get(mf, rule, &value, &mask);
-
-        byte_ofs = mf->n_bytes - ofs / 8;
-        vp = &((uint8_t *) &value)[byte_ofs];
-        mp = &((uint8_t *) &mask)[byte_ofs];
-        if (ofs % 8) {
-            unsigned int chunk = MIN(8 - ofs % 8, n_bits);
-            uint8_t chunk_mask = ((1 << chunk) - 1) << (ofs % 8);
-
-            *--vp &= ~chunk_mask;
-            *vp   |= chunk_mask & (x << (ofs % 8));
-            *--mp |= chunk_mask;
+        return 0;
+    }
 
-            x >>= chunk;
-            n_bits -= chunk;
-            ofs += chunk;
-        }
-        while (n_bits >= 8) {
-            *--vp = x;
-            *--mp = 0xff;
-            x >>= 8;
-            n_bits -= 8;
-            ofs += 8;
-        }
-        if (n_bits) {
-            uint8_t chunk_mask = (1 << n_bits) - 1;
+    return OFPERR_OFPBAC_BAD_ARGUMENT;
+}
 
-            *--vp &= ~chunk_mask;
-            *vp   |= chunk_mask & x;
-            *--mp |= chunk_mask;
-        }
+/* Checks whether 'sf' is valid for reading a subfield out of 'flow'.  Returns
+ * 0 if so, otherwise an OpenFlow error code (e.g. as returned by
+ * ofp_mkerr()).  */
+enum ofperr
+mf_check_src(const struct mf_subfield *sf, const struct flow *flow)
+{
+    return mf_check__(sf, flow, "source");
+}
 
-        mf_set(mf, &value, &mask, rule);
+/* Checks whether 'sf' is valid for writing a subfield into 'flow'.  Returns 0
+ * if so, otherwise an OpenFlow error code (e.g. as returned by
+ * ofp_mkerr()). */
+enum ofperr
+mf_check_dst(const struct mf_subfield *sf, const struct flow *flow)
+{
+    int error = mf_check__(sf, flow, "destination");
+    if (!error && !sf->field->writable) {
+        VLOG_WARN_RL(&rl, "destination field %s is not writable",
+                     sf->field->name);
+        return OFPERR_OFPBAC_BAD_ARGUMENT;
     }
+    return error;
 }
 
 /* Copies the value and wildcard bit pattern for 'mf' from 'rule' into the
@@ -2141,3 +2125,222 @@ mf_format(const struct mf_field *mf,
         NOT_REACHED();
     }
 }
+
+/* Makes a subfield starting at bit offset 'ofs' and continuing for 'n_bits' in
+ * 'rule''s field 'mf' exactly match the 'n_bits' least-significant bits of
+ * 'x'.
+ *
+ * Example: suppose that 'mf' is originally the following 2-byte field in
+ * 'rule':
+ *
+ *     value == 0xe00a == 2#1110000000001010
+ *      mask == 0xfc3f == 2#1111110000111111
+ *
+ * The call mf_set_subfield(mf, 0x55, 8, 7, rule) would have the following
+ * effect (note that 0x55 is 2#1010101):
+ *
+ *     value == 0xd50a == 2#1101010100001010
+ *      mask == 0xff3f == 2#1111111100111111
+ *
+ * The caller is responsible for ensuring that the result will be a valid
+ * wildcard pattern for 'mf'.  The caller is responsible for ensuring that
+ * 'rule' meets 'mf''s prerequisites. */
+void
+mf_set_subfield(const struct mf_subfield *sf, uint64_t x,
+                struct cls_rule *rule)
+{
+    const struct mf_field *field = sf->field;
+    unsigned int n_bits = sf->n_bits;
+    unsigned int ofs = sf->ofs;
+
+    if (ofs == 0 && field->n_bytes * 8 == n_bits) {
+        union mf_value value;
+        int i;
+
+        for (i = field->n_bytes - 1; i >= 0; i--) {
+            ((uint8_t *) &value)[i] = x;
+            x >>= 8;
+        }
+        mf_set_value(field, &value, rule);
+    } else {
+        union mf_value value, mask;
+        uint8_t *vp = (uint8_t *) &value;
+        uint8_t *mp = (uint8_t *) &mask;
+
+        mf_get(field, rule, &value, &mask);
+        bitwise_put(x,          vp, field->n_bytes, ofs, n_bits);
+        bitwise_put(UINT64_MAX, mp, field->n_bytes, ofs, n_bits);
+        mf_set(field, &value, &mask, rule);
+    }
+}
+
+/* Similar to mf_set_subfield() but modifies only a flow, not a cls_rule. */
+void
+mf_set_subfield_value(const struct mf_subfield *sf, uint64_t x,
+                      struct flow *flow)
+{
+    const struct mf_field *field = sf->field;
+    unsigned int n_bits = sf->n_bits;
+    unsigned int ofs = sf->ofs;
+    union mf_value value;
+
+    if (ofs == 0 && field->n_bytes * 8 == n_bits) {
+        int i;
+
+        for (i = field->n_bytes - 1; i >= 0; i--) {
+            ((uint8_t *) &value)[i] = x;
+            x >>= 8;
+        }
+        mf_set_flow_value(field, &value, flow);
+    } else {
+        mf_get_value(field, flow, &value);
+        bitwise_put(x, &value, field->n_bytes, ofs, n_bits);
+        mf_set_flow_value(field, &value, flow);
+    }
+}
+
+/* Returns the value of 'sf' within 'flow'.  'sf' must be valid for reading
+ * 'flow', e.g. as checked by mf_check_src() and sf->n_bits must be 64 or
+ * less. */
+uint64_t
+mf_get_subfield(const struct mf_subfield *sf, const struct flow *flow)
+{
+    union mf_value value;
+
+    mf_get_value(sf->field, flow, &value);
+    return bitwise_get(&value, sf->field->n_bytes, sf->ofs, sf->n_bits);
+}
+
+/* Formats 'sf' into 's' in a format normally acceptable to
+ * mf_parse_subfield().  (It won't be acceptable if sf->field is NULL or if
+ * sf->field has no NXM name.) */
+void
+mf_format_subfield(const struct mf_subfield *sf, struct ds *s)
+{
+    if (!sf->field) {
+        ds_put_cstr(s, "<unknown>");
+    } else if (sf->field->nxm_name) {
+        ds_put_cstr(s, sf->field->nxm_name);
+    } else if (sf->field->nxm_header) {
+        uint32_t header = sf->field->nxm_header;
+        ds_put_format(s, "%d:%d", NXM_VENDOR(header), NXM_FIELD(header));
+    } else {
+        ds_put_cstr(s, sf->field->name);
+    }
+
+    if (sf->ofs == 0 && sf->n_bits == sf->field->n_bits) {
+        ds_put_cstr(s, "[]");
+    } else if (sf->n_bits == 1) {
+        ds_put_format(s, "[%d]", sf->ofs);
+    } else {
+        ds_put_format(s, "[%d..%d]", sf->ofs, sf->ofs + sf->n_bits - 1);
+    }
+}
+
+static const struct mf_field *
+mf_parse_subfield_name(const char *name, int name_len, bool *wild)
+{
+    int i;
+
+    *wild = name_len > 2 && !memcmp(&name[name_len - 2], "_W", 2);
+    if (*wild) {
+        name_len -= 2;
+    }
+
+    for (i = 0; i < MFF_N_IDS; i++) {
+        const struct mf_field *mf = mf_from_id(i);
+
+        if (mf->nxm_name
+            && !strncmp(mf->nxm_name, name, name_len)
+            && mf->nxm_name[name_len] == '\0') {
+            return mf;
+        }
+    }
+
+    return NULL;
+}
+
+/* Parses a subfield from the beginning of '*sp' into 'sf'.  If successful,
+ * returns NULL and advances '*sp' to the first byte following the parsed
+ * string.  On failure, returns a malloc()'d error message, does not modify
+ * '*sp', and does not properly initialize 'sf'.
+ *
+ * The syntax parsed from '*sp' takes the form "header[start..end]" where
+ * 'header' is the name of an NXM field and 'start' and 'end' are (inclusive)
+ * bit indexes.  "..end" may be omitted to indicate a single bit.  "start..end"
+ * may both be omitted (the [] are still required) to indicate an entire
+ * field. */
+char *
+mf_parse_subfield__(struct mf_subfield *sf, const char **sp)
+{
+    const struct mf_field *field;
+    const char *name;
+    int start, end;
+    const char *s;
+    int name_len;
+    bool wild;
+
+    s = *sp;
+    name = s;
+    name_len = strcspn(s, "[");
+    if (s[name_len] != '[') {
+        return xasprintf("%s: missing [ looking for field name", *sp);
+    }
+
+    field = mf_parse_subfield_name(name, name_len, &wild);
+    if (!field) {
+        return xasprintf("%s: unknown field `%.*s'", *sp, name_len, s);
+    }
+
+    s += name_len;
+    if (sscanf(s, "[%d..%d]", &start, &end) == 2) {
+        /* Nothing to do. */
+    } else if (sscanf(s, "[%d]", &start) == 1) {
+        end = start;
+    } else if (!strncmp(s, "[]", 2)) {
+        start = 0;
+        end = field->n_bits - 1;
+    } else {
+        return xasprintf("%s: syntax error expecting [] or [<bit>] or "
+                         "[<start>..<end>]", *sp);
+    }
+    s = strchr(s, ']') + 1;
+
+    if (start > end) {
+        return xasprintf("%s: starting bit %d is after ending bit %d",
+                         *sp, start, end);
+    } else if (start >= field->n_bits) {
+        return xasprintf("%s: starting bit %d is not valid because field is "
+                         "only %d bits wide", *sp, start, field->n_bits);
+    } else if (end >= field->n_bits){
+        return xasprintf("%s: ending bit %d is not valid because field is "
+                         "only %d bits wide", *sp, end, field->n_bits);
+    }
+
+    sf->field = field;
+    sf->ofs = start;
+    sf->n_bits = end - start + 1;
+
+    *sp = s;
+    return NULL;
+}
+
+/* Parses a subfield from the beginning of 's' into 'sf'.  Returns the first
+ * byte in 's' following the parsed string.
+ *
+ * Exits with an error message if 's' has incorrect syntax.
+ *
+ * The syntax parsed from 's' takes the form "header[start..end]" where
+ * 'header' is the name of an NXM field and 'start' and 'end' are (inclusive)
+ * bit indexes.  "..end" may be omitted to indicate a single bit.  "start..end"
+ * may both be omitted (the [] are still required) to indicate an entire
+ * field.  */
+const char *
+mf_parse_subfield(struct mf_subfield *sf, const char *s)
+{
+    char *msg = mf_parse_subfield__(sf, &s);
+    if (msg) {
+        ovs_fatal(0, "%s", msg);
+    }
+    return s;
+}
diff --git a/lib/meta-flow.h b/lib/meta-flow.h
index a552e3ce..49178347 100644
--- a/lib/meta-flow.h
+++ b/lib/meta-flow.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 Nicira Networks.
+ * Copyright (c) 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@
 #include <netinet/in.h>
 #include <netinet/ip6.h>
 #include "flow.h"
+#include "ofp-errors.h"
 #include "packets.h"
 
 struct cls_rule;
@@ -198,6 +199,13 @@ union mf_value {
     struct in6_addr ipv6;
 };
 
+/* Part of a field. */
+struct mf_subfield {
+    const struct mf_field *field;
+    unsigned int ofs;           /* Bit offset. */
+    unsigned int n_bits;        /* Number of bits. */
+};
+
 /* Finding mf_fields. */
 const struct mf_field *mf_from_id(enum mf_field_id);
 const struct mf_field *mf_from_name(const char *name);
@@ -230,13 +238,25 @@ void mf_get(const struct mf_field *, const struct cls_rule *,
 void mf_set(const struct mf_field *,
             const union mf_value *value, const union mf_value *mask,
             struct cls_rule *);
-void mf_set_subfield(const struct mf_field *, uint64_t value, unsigned int ofs,
-                     unsigned int n_bits, struct cls_rule *);
 
 void mf_set_wild(const struct mf_field *, struct cls_rule *);
 
 void mf_random_value(const struct mf_field *, union mf_value *value);
 
+/* Subfields. */
+void mf_set_subfield(const struct mf_subfield *, uint64_t value,
+                     struct cls_rule *);
+void mf_set_subfield_value(const struct mf_subfield *, uint64_t value,
+                           struct flow *);
+uint64_t mf_get_subfield(const struct mf_subfield *, const struct flow *);
+
+void mf_format_subfield(const struct mf_subfield *, struct ds *);
+char *mf_parse_subfield__(struct mf_subfield *sf, const char **s);
+const char *mf_parse_subfield(struct mf_subfield *, const char *);
+
+enum ofperr mf_check_src(const struct mf_subfield *, const struct flow *);
+enum ofperr mf_check_dst(const struct mf_subfield *, const struct flow *);
+
 /* Parsing and formatting. */
 char *mf_parse(const struct mf_field *, const char *,
                union mf_value *value, union mf_value *mask);
diff --git a/lib/multipath.c b/lib/multipath.c
index 51758033..46976fb2 100644
--- a/lib/multipath.c
+++ b/lib/multipath.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010, 2011 Nicira Networks.
+ * Copyright (c) 2010, 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@
 #include <sys/types.h>
 #include <netinet/in.h>
 #include "dynamic-string.h"
+#include "meta-flow.h"
 #include "nx-match.h"
 #include "ofp-errors.h"
 #include "ofp-util.h"
@@ -39,11 +40,11 @@ multipath_check(const struct nx_action_multipath *mp, const struct flow *flow)
 {
     uint32_t n_links = ntohs(mp->max_link) + 1;
     size_t min_n_bits = log_2_ceil(n_links);
-    int ofs = nxm_decode_ofs(mp->ofs_nbits);
-    int n_bits = nxm_decode_n_bits(mp->ofs_nbits);
+    struct mf_subfield dst;
     enum ofperr error;
 
-    error = nxm_dst_check(mp->dst, ofs, n_bits, flow);
+    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
+    error = mf_check_dst(&dst, flow);
     if (error) {
         return error;
     }
@@ -56,7 +57,7 @@ multipath_check(const struct nx_action_multipath *mp, const struct flow *flow)
                && mp->algorithm != htons(NX_MP_ALG_ITER_HASH)) {
         VLOG_WARN_RL(&rl, "unsupported algorithm %"PRIu16,
                      ntohs(mp->algorithm));
-    } else if (n_bits < min_n_bits) {
+    } else if (dst.n_bits < min_n_bits) {
         VLOG_WARN_RL(&rl, "multipath action requires at least %zu bits for "
                      "%"PRIu32" links", min_n_bits, n_links);
     } else {
@@ -80,8 +81,10 @@ multipath_execute(const struct nx_action_multipath *mp, struct flow *flow)
     uint16_t link = multipath_algorithm(hash, ntohs(mp->algorithm),
                                         ntohs(mp->max_link) + 1,
                                         ntohl(mp->arg));
+    struct mf_subfield dst;
 
-    nxm_reg_load(mp->dst, mp->ofs_nbits, link, flow);
+    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
+    mf_set_subfield_value(&dst, link, flow);
 }
 
 static uint16_t
@@ -166,9 +169,8 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
 {
     char *s = xstrdup(s_);
     char *save_ptr = NULL;
-    char *fields, *basis, *algorithm, *n_links_str, *arg, *dst;
-    uint32_t header;
-    int ofs, n_bits;
+    char *fields, *basis, *algorithm, *n_links_str, *arg, *dst_s;
+    struct mf_subfield dst;
     int n_links;
 
     fields = strtok_r(s, ", ", &save_ptr);
@@ -176,8 +178,8 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
     algorithm = strtok_r(NULL, ", ", &save_ptr);
     n_links_str = strtok_r(NULL, ", ", &save_ptr);
     arg = strtok_r(NULL, ", ", &save_ptr);
-    dst = strtok_r(NULL, ", ", &save_ptr);
-    if (!dst) {
+    dst_s = strtok_r(NULL, ", ", &save_ptr);
+    if (!dst_s) {
         ovs_fatal(0, "%s: not enough arguments to multipath action", s_);
     }
 
@@ -209,14 +211,14 @@ multipath_parse(struct nx_action_multipath *mp, const char *s_)
     mp->max_link = htons(n_links - 1);
     mp->arg = htonl(atoi(arg));
 
-    nxm_parse_field_bits(dst, &header, &ofs, &n_bits);
-    if (n_bits < 16 && n_links > (1u << n_bits)) {
+    mf_parse_subfield(&dst, dst_s);
+    if (dst.n_bits < 16 && n_links > (1u << dst.n_bits)) {
         ovs_fatal(0, "%s: %d-bit destination field has %u possible values, "
                   "less than specified n_links %d",
-                  s_, n_bits, 1u << n_bits, n_links);
+                  s_, dst.n_bits, 1u << dst.n_bits, n_links);
     }
-    mp->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
-    mp->dst = htonl(header);
+    mp->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
+    mp->dst = htonl(dst.field->nxm_header);
 
     free(s);
 }
@@ -229,6 +231,8 @@ multipath_format(const struct nx_action_multipath *mp, struct ds *s)
     uint16_t mp_fields    = ntohs(mp->fields);
     uint16_t mp_algorithm = ntohs(mp->algorithm);
 
+    struct mf_subfield dst;
+
     fields = flow_hash_fields_to_str(mp_fields);
 
     switch ((enum nx_mp_algorithm) mp_algorithm) {
@@ -251,7 +255,7 @@ multipath_format(const struct nx_action_multipath *mp, struct ds *s)
     ds_put_format(s, "multipath(%s,%"PRIu16",%s,%d,%"PRIu16",",
                   fields, ntohs(mp->basis), algorithm, ntohs(mp->max_link) + 1,
                   ntohl(mp->arg));
-    nxm_format_field_bits(s, ntohl(mp->dst), nxm_decode_ofs(mp->ofs_nbits),
-                          nxm_decode_n_bits(mp->ofs_nbits));
+    nxm_decode(&dst, mp->dst, mp->ofs_nbits);
+    mf_format_subfield(&dst, s);
     ds_put_char(s, ')');
 }
diff --git a/lib/nx-match.c b/lib/nx-match.c
index ef166c79..9d3b2be1 100644
--- a/lib/nx-match.c
+++ b/lib/nx-match.c
@@ -794,97 +794,40 @@ nx_match_from_string(const char *s, struct ofpbuf *b)
     return match_len;
 }
 
-const char *
-nxm_parse_field_bits(const char *s, uint32_t *headerp, int *ofsp, int *n_bitsp)
-{
-    const char *full_s = s;
-    const char *name;
-    uint32_t header;
-    int start, end;
-    int name_len;
-    int width;
-
-    name = s;
-    name_len = strcspn(s, "[");
-    if (s[name_len] != '[') {
-        ovs_fatal(0, "%s: missing [ looking for field name", full_s);
-    }
-
-    header = parse_nxm_field_name(name, name_len);
-    if (!header) {
-        ovs_fatal(0, "%s: unknown field `%.*s'", full_s, name_len, s);
-    }
-    width = nxm_field_bits(header);
-
-    s += name_len;
-    if (sscanf(s, "[%d..%d]", &start, &end) == 2) {
-        /* Nothing to do. */
-    } else if (sscanf(s, "[%d]", &start) == 1) {
-        end = start;
-    } else if (!strncmp(s, "[]", 2)) {
-        start = 0;
-        end = width - 1;
-    } else {
-        ovs_fatal(0, "%s: syntax error expecting [] or [<bit>] or "
-                  "[<start>..<end>]", full_s);
-    }
-    s = strchr(s, ']') + 1;
-
-    if (start > end) {
-        ovs_fatal(0, "%s: starting bit %d is after ending bit %d",
-                  full_s, start, end);
-    } else if (start >= width) {
-        ovs_fatal(0, "%s: starting bit %d is not valid because field is only "
-                  "%d bits wide", full_s, start, width);
-    } else if (end >= width){
-        ovs_fatal(0, "%s: ending bit %d is not valid because field is only "
-                  "%d bits wide", full_s, end, width);
-    }
-
-    *headerp = header;
-    *ofsp = start;
-    *n_bitsp = end - start + 1;
-
-    return s;
-}
-
 void
 nxm_parse_reg_move(struct nx_action_reg_move *move, const char *s)
 {
     const char *full_s = s;
-    uint32_t src, dst;
-    int src_ofs, dst_ofs;
-    int src_n_bits, dst_n_bits;
+    struct mf_subfield src, dst;
 
-    s = nxm_parse_field_bits(s, &src, &src_ofs, &src_n_bits);
+    s = mf_parse_subfield(&src, s);
     if (strncmp(s, "->", 2)) {
         ovs_fatal(0, "%s: missing `->' following source", full_s);
     }
     s += 2;
-    s = nxm_parse_field_bits(s, &dst, &dst_ofs, &dst_n_bits);
+    s = mf_parse_subfield(&dst, s);
     if (*s != '\0') {
         ovs_fatal(0, "%s: trailing garbage following destination", full_s);
     }
 
-    if (src_n_bits != dst_n_bits) {
+    if (src.n_bits != dst.n_bits) {
         ovs_fatal(0, "%s: source field is %d bits wide but destination is "
-                  "%d bits wide", full_s, src_n_bits, dst_n_bits);
+                  "%d bits wide", full_s, src.n_bits, dst.n_bits);
     }
 
     ofputil_init_NXAST_REG_MOVE(move);
-    move->n_bits = htons(src_n_bits);
-    move->src_ofs = htons(src_ofs);
-    move->dst_ofs = htons(dst_ofs);
-    move->src = htonl(src);
-    move->dst = htonl(dst);
+    move->n_bits = htons(src.n_bits);
+    move->src_ofs = htons(src.ofs);
+    move->dst_ofs = htons(dst.ofs);
+    move->src = htonl(src.field->nxm_header);
+    move->dst = htonl(dst.field->nxm_header);
 }
 
 void
 nxm_parse_reg_load(struct nx_action_reg_load *load, const char *s)
 {
     const char *full_s = s;
-    uint32_t dst;
-    int ofs, n_bits;
+    struct mf_subfield dst;
     uint64_t value;
 
     value = strtoull(s, (char **) &s, 0);
@@ -892,151 +835,85 @@ nxm_parse_reg_load(struct nx_action_reg_load *load, const char *s)
         ovs_fatal(0, "%s: missing `->' following value", full_s);
     }
     s += 2;
-    s = nxm_parse_field_bits(s, &dst, &ofs, &n_bits);
+    s = mf_parse_subfield(&dst, s);
     if (*s != '\0') {
         ovs_fatal(0, "%s: trailing garbage following destination", full_s);
     }
 
-    if (n_bits < 64 && (value >> n_bits) != 0) {
-        ovs_fatal(0, "%s: value %"PRIu64" does not fit into %d bits",
-                  full_s, value, n_bits);
+    if (dst.n_bits < 64 && (value >> dst.n_bits) != 0) {
+        ovs_fatal(0, "%s: value %"PRIu64" does not fit into %u bits",
+                  full_s, value, dst.n_bits);
     }
 
     ofputil_init_NXAST_REG_LOAD(load);
-    load->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
-    load->dst = htonl(dst);
+    load->ofs_nbits = nxm_encode_ofs_nbits(dst.ofs, dst.n_bits);
+    load->dst = htonl(dst.field->nxm_header);
     load->value = htonll(value);
 }
 
 /* nxm_format_reg_move(), nxm_format_reg_load(). */
 
-void
-nxm_format_field_bits(struct ds *s, uint32_t header, int ofs, int n_bits)
-{
-    format_nxm_field_name(s, header);
-    if (ofs == 0 && n_bits == nxm_field_bits(header)) {
-        ds_put_cstr(s, "[]");
-    } else if (n_bits == 1) {
-        ds_put_format(s, "[%d]", ofs);
-    } else {
-        ds_put_format(s, "[%d..%d]", ofs, ofs + n_bits - 1);
-    }
-}
-
 void
 nxm_format_reg_move(const struct nx_action_reg_move *move, struct ds *s)
 {
-    int n_bits = ntohs(move->n_bits);
-    int src_ofs = ntohs(move->src_ofs);
-    int dst_ofs = ntohs(move->dst_ofs);
-    uint32_t src = ntohl(move->src);
-    uint32_t dst = ntohl(move->dst);
+    struct mf_subfield src, dst;
+
+    nxm_decode_discrete(&src, move->src, move->src_ofs, move->n_bits);
+    nxm_decode_discrete(&dst, move->dst, move->dst_ofs, move->n_bits);
 
     ds_put_format(s, "move:");
-    nxm_format_field_bits(s, src, src_ofs, n_bits);
+    mf_format_subfield(&src, s);
     ds_put_cstr(s, "->");
-    nxm_format_field_bits(s, dst, dst_ofs, n_bits);
+    mf_format_subfield(&dst, s);
 }
 
 void
 nxm_format_reg_load(const struct nx_action_reg_load *load, struct ds *s)
 {
-    int ofs = nxm_decode_ofs(load->ofs_nbits);
-    int n_bits = nxm_decode_n_bits(load->ofs_nbits);
-    uint32_t dst = ntohl(load->dst);
-    uint64_t value = ntohll(load->value);
+    struct mf_subfield dst;
 
-    ds_put_format(s, "load:%#"PRIx64"->", value);
-    nxm_format_field_bits(s, dst, ofs, n_bits);
+    ds_put_format(s, "load:%#"PRIx64"->", ntohll(load->value));
+
+    nxm_decode(&dst, load->dst, load->ofs_nbits);
+    mf_format_subfield(&dst, s);
 }
 
 /* nxm_check_reg_move(), nxm_check_reg_load(). */
 
-static bool
-field_ok(const struct mf_field *mf, const struct flow *flow, int size)
-{
-    return (mf
-            && mf_are_prereqs_ok(mf, flow)
-            && size <= nxm_field_bits(mf->nxm_header));
-}
-
-int
+enum ofperr
 nxm_check_reg_move(const struct nx_action_reg_move *action,
                    const struct flow *flow)
 {
-    int src_ofs, dst_ofs, n_bits;
+    struct mf_subfield src;
+    struct mf_subfield dst;
     int error;
 
-    n_bits = ntohs(action->n_bits);
-    src_ofs = ntohs(action->src_ofs);
-    dst_ofs = ntohs(action->dst_ofs);
-
-    error = nxm_src_check(action->src, src_ofs, n_bits, flow);
+    nxm_decode_discrete(&src, action->src, action->src_ofs, action->n_bits);
+    error = mf_check_src(&src, flow);
     if (error) {
         return error;
     }
 
-    return nxm_dst_check(action->dst, dst_ofs, n_bits, flow);
-}
-
-/* Given a flow, checks that the source field represented by 'src_header'
- * in the range ['ofs', 'ofs' + 'n_bits') is valid. */
-enum ofperr
-nxm_src_check(ovs_be32 src_header_, unsigned int ofs, unsigned int n_bits,
-              const struct flow *flow)
-{
-    uint32_t src_header = ntohl(src_header_);
-    const struct mf_field *src = mf_from_nxm_header(src_header);
-
-    if (!n_bits) {
-        VLOG_WARN_RL(&rl, "zero bit source field");
-    } else if (NXM_HASMASK(src_header) || !field_ok(src, flow, ofs + n_bits)) {
-        VLOG_WARN_RL(&rl, "invalid source field");
-    } else {
-        return 0;
-    }
-
-    return OFPERR_OFPBAC_BAD_ARGUMENT;
-}
-
-/* Given a flow, checks that the destination field represented by 'dst_header'
- * in the range ['ofs', 'ofs' + 'n_bits') is valid. */
-enum ofperr
-nxm_dst_check(ovs_be32 dst_header_, unsigned int ofs, unsigned int n_bits,
-              const struct flow *flow)
-{
-    uint32_t dst_header = ntohl(dst_header_);
-    const struct mf_field *dst = mf_from_nxm_header(dst_header);
-
-    if (!n_bits) {
-        VLOG_WARN_RL(&rl, "zero bit destination field");
-    } else if (NXM_HASMASK(dst_header) || !field_ok(dst, flow, ofs + n_bits)) {
-        VLOG_WARN_RL(&rl, "invalid destination field");
-    } else if (!dst->writable) {
-        VLOG_WARN_RL(&rl, "destination field is not writable");
-    } else {
-        return 0;
-    }
-
-    return OFPERR_OFPBAC_BAD_ARGUMENT;
+    nxm_decode_discrete(&dst, action->dst, action->dst_ofs, action->n_bits);
+    return mf_check_dst(&dst, flow);
 }
 
 enum ofperr
 nxm_check_reg_load(const struct nx_action_reg_load *action,
                    const struct flow *flow)
 {
-    unsigned int ofs = nxm_decode_ofs(action->ofs_nbits);
-    unsigned int n_bits = nxm_decode_n_bits(action->ofs_nbits);
+    struct mf_subfield dst;
     enum ofperr error;
 
-    error = nxm_dst_check(action->dst, ofs, n_bits, flow);
+    nxm_decode(&dst, action->dst, action->ofs_nbits);
+    error = mf_check_dst(&dst, flow);
     if (error) {
         return error;
     }
 
     /* Reject 'action' if a bit numbered 'n_bits' or higher is set to 1 in
      * action->value. */
-    if (n_bits < 64 && ntohll(action->value) >> n_bits) {
+    if (dst.n_bits < 64 && ntohll(action->value) >> dst.n_bits) {
         return OFPERR_OFPBAC_BAD_ARGUMENT;
     }
 
@@ -1045,64 +922,72 @@ nxm_check_reg_load(const struct nx_action_reg_load *action,
 
 /* nxm_execute_reg_move(), nxm_execute_reg_load(). */
 
-/* Returns the value of the NXM field corresponding to 'header' at 'ofs_nbits'
- * in 'flow'. */
-uint64_t
-nxm_read_field_bits(ovs_be32 header, ovs_be16 ofs_nbits,
-                    const struct flow *flow)
-{
-    const struct mf_field *field = mf_from_nxm_header(ntohl(header));
-    union mf_value value;
-    union mf_value bits;
-
-    mf_get_value(field, flow, &value);
-    bits.be64 = htonll(0);
-    bitwise_copy(&value, field->n_bytes, nxm_decode_ofs(ofs_nbits),
-                 &bits, sizeof bits.be64, 0,
-                 nxm_decode_n_bits(ofs_nbits));
-    return ntohll(bits.be64);
-}
-
 void
 nxm_execute_reg_move(const struct nx_action_reg_move *action,
                      struct flow *flow)
 {
-    const struct mf_field *src = mf_from_nxm_header(ntohl(action->src));
-    const struct mf_field *dst = mf_from_nxm_header(ntohl(action->dst));
+    struct mf_subfield src, dst;
     union mf_value src_value;
     union mf_value dst_value;
 
-    mf_get_value(dst, flow, &dst_value);
-    mf_get_value(src, flow, &src_value);
-    bitwise_copy(&src_value, src->n_bytes, ntohs(action->src_ofs),
-                 &dst_value, dst->n_bytes, ntohs(action->dst_ofs),
-                 ntohs(action->n_bits));
-    mf_set_flow_value(dst, &dst_value, flow);
+    nxm_decode_discrete(&src, action->src, action->src_ofs, action->n_bits);
+    nxm_decode_discrete(&dst, action->dst, action->dst_ofs, action->n_bits);
+
+    mf_get_value(dst.field, flow, &dst_value);
+    mf_get_value(src.field, flow, &src_value);
+    bitwise_copy(&src_value, src.field->n_bytes, src.ofs,
+                 &dst_value, dst.field->n_bytes, dst.ofs,
+                 src.n_bits);
+    mf_set_flow_value(dst.field, &dst_value, flow);
 }
 
 void
 nxm_execute_reg_load(const struct nx_action_reg_load *action,
                      struct flow *flow)
 {
-    nxm_reg_load(action->dst, action->ofs_nbits, ntohll(action->value), flow);
+    struct mf_subfield dst;
+
+    nxm_decode(&dst, action->dst, action->ofs_nbits);
+    mf_set_subfield_value(&dst, ntohll(action->value), flow);
 }
 
-/* Calculates ofs and n_bits from the given 'ofs_nbits' parameter, and copies
- * 'src_data'[0:n_bits] to 'dst_header'[ofs:ofs+n_bits] in the given 'flow'. */
+/* Initializes 'sf->field' with the field corresponding to the given NXM
+ * 'header' and 'sf->ofs' and 'sf->n_bits' decoded from 'ofs_nbits' with
+ * nxm_decode_ofs() and nxm_decode_n_bits(), respectively.
+ *
+ * Afterward, 'sf' might be invalid in a few different ways:
+ *
+ *   - 'sf->field' will be NULL if 'header' is unknown.
+ *
+ *   - 'sf->ofs' and 'sf->n_bits' might exceed the width of sf->field.
+ *
+ * The caller should call mf_check_src() or mf_check_dst() to check for these
+ * problems. */
 void
-nxm_reg_load(ovs_be32 dst_header, ovs_be16 ofs_nbits, uint64_t src_data,
-             struct flow *flow)
+nxm_decode(struct mf_subfield *sf, ovs_be32 header, ovs_be16 ofs_nbits)
 {
-    const struct mf_field *dst = mf_from_nxm_header(ntohl(dst_header));
-    int n_bits = nxm_decode_n_bits(ofs_nbits);
-    int dst_ofs = nxm_decode_ofs(ofs_nbits);
-    union mf_value dst_value;
-    union mf_value src_value;
+    sf->field = mf_from_nxm_header(ntohl(header));
+    sf->ofs = nxm_decode_ofs(ofs_nbits);
+    sf->n_bits = nxm_decode_n_bits(ofs_nbits);
+}
 
-    mf_get_value(dst, flow, &dst_value);
-    src_value.be64 = htonll(src_data);
-    bitwise_copy(&src_value, sizeof src_value.be64, 0,
-                 &dst_value, dst->n_bytes, dst_ofs,
-                 n_bits);
-    mf_set_flow_value(dst, &dst_value, flow);
+/* Initializes 'sf->field' with the field corresponding to the given NXM
+ * 'header' and 'sf->ofs' and 'sf->n_bits' from 'ofs' and 'n_bits',
+ * respectively.
+ *
+ * Afterward, 'sf' might be invalid in a few different ways:
+ *
+ *   - 'sf->field' will be NULL if 'header' is unknown.
+ *
+ *   - 'sf->ofs' and 'sf->n_bits' might exceed the width of sf->field.
+ *
+ * The caller should call mf_check_src() or mf_check_dst() to check for these
+ * problems. */
+void
+nxm_decode_discrete(struct mf_subfield *sf, ovs_be32 header,
+                    ovs_be16 ofs, ovs_be16 n_bits)
+{
+    sf->field = mf_from_nxm_header(ntohl(header));
+    sf->ofs = ntohs(ofs);
+    sf->n_bits = ntohs(n_bits);
 }
diff --git a/lib/nx-match.h b/lib/nx-match.h
index c1892873..3406e046 100644
--- a/lib/nx-match.h
+++ b/lib/nx-match.h
@@ -26,6 +26,7 @@
 struct cls_rule;
 struct ds;
 struct flow;
+struct mf_subfield;
 struct ofpbuf;
 struct nx_action_reg_load;
 struct nx_action_reg_move;
@@ -47,37 +48,24 @@ int nx_put_match(struct ofpbuf *, const struct cls_rule *,
 char *nx_match_to_string(const uint8_t *, unsigned int match_len);
 int nx_match_from_string(const char *, struct ofpbuf *);
 
-uint64_t nxm_read_field_bits(ovs_be32 header, ovs_be16 ofs_nbits,
-                             const struct flow *);
-
 void nxm_parse_reg_move(struct nx_action_reg_move *, const char *);
 void nxm_parse_reg_load(struct nx_action_reg_load *, const char *);
 
 void nxm_format_reg_move(const struct nx_action_reg_move *, struct ds *);
 void nxm_format_reg_load(const struct nx_action_reg_load *, struct ds *);
 
-int nxm_check_reg_move(const struct nx_action_reg_move *, const struct flow *);
+enum ofperr nxm_check_reg_move(const struct nx_action_reg_move *,
+                               const struct flow *);
 enum ofperr nxm_check_reg_load(const struct nx_action_reg_load *,
                                const struct flow *);
-enum ofperr nxm_src_check(ovs_be32 src, unsigned int ofs, unsigned int n_bits,
-                          const struct flow *);
-enum ofperr nxm_dst_check(ovs_be32 dst, unsigned int ofs, unsigned int n_bits,
-                          const struct flow *);
 
 void nxm_execute_reg_move(const struct nx_action_reg_move *, struct flow *);
 void nxm_execute_reg_load(const struct nx_action_reg_load *, struct flow *);
-void nxm_reg_load(ovs_be32 dst, ovs_be16 ofs_nbits, uint64_t src_data,
-                  struct flow *);
 
 int nxm_field_bytes(uint32_t header);
 int nxm_field_bits(uint32_t header);
 
-const char *nxm_parse_field_bits(const char *s,
-                                 uint32_t *headerp, int *ofsp, int *n_bitsp);
-void nxm_format_field_bits(struct ds *, uint32_t header, int ofs, int n_bits);
-
-/* Dealing with the 'ofs_nbits' members of struct nx_action_reg_load and struct
- * nx_action_multipath. */
+/* Dealing with the 'ofs_nbits' members in several Nicira extensions. */
 
 static inline ovs_be16
 nxm_encode_ofs_nbits(int ofs, int n_bits)
@@ -96,6 +84,10 @@ nxm_decode_n_bits(ovs_be16 ofs_nbits)
 {
     return (ntohs(ofs_nbits) & 0x3f) + 1;
 }
+
+void nxm_decode(struct mf_subfield *, ovs_be32 header, ovs_be16 ofs_nbits);
+void nxm_decode_discrete(struct mf_subfield *, ovs_be32 header,
+                         ovs_be16 ofs, ovs_be16 n_bits);
 
 /* Upper bound on the length of an nx_match.  The longest nx_match (an
  * IPV6 neighbor discovery message using 5 registers) would be:
diff --git a/lib/ofp-parse.c b/lib/ofp-parse.c
index 5321364b..3a1fcb7d 100644
--- a/lib/ofp-parse.c
+++ b/lib/ofp-parse.c
@@ -151,14 +151,13 @@ parse_output(struct ofpbuf *b, char *arg)
 {
     if (strchr(arg, '[')) {
         struct nx_action_output_reg *naor;
-        int ofs, n_bits;
-        uint32_t src;
+        struct mf_subfield src;
 
-        nxm_parse_field_bits(arg, &src, &ofs, &n_bits);
+        mf_parse_subfield(&src, arg);
 
         naor = ofputil_put_NXAST_OUTPUT_REG(b);
-        naor->ofs_nbits = nxm_encode_ofs_nbits(ofs, n_bits);
-        naor->src = htonl(src);
+        naor->ofs_nbits = nxm_encode_ofs_nbits(src.ofs, src.n_bits);
+        naor->src = htonl(src.field->nxm_header);
         naor->max_len = htons(UINT16_MAX);
     } else {
         put_output_action(b, str_to_u32(arg));
diff --git a/lib/ofp-print.c b/lib/ofp-print.c
index 7bc26c9d..3c5c34a3 100644
--- a/lib/ofp-print.c
+++ b/lib/ofp-print.c
@@ -33,6 +33,7 @@
 #include "flow.h"
 #include "learn.h"
 #include "multipath.h"
+#include "meta-flow.h"
 #include "nx-match.h"
 #include "ofp-errors.h"
 #include "ofp-util.h"
@@ -187,6 +188,7 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
     const struct nx_action_multipath *nam;
     const struct nx_action_autopath *naa;
     const struct nx_action_output_reg *naor;
+    struct mf_subfield subfield;
     uint16_t port;
 
     switch (code) {
@@ -319,9 +321,8 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
     case OFPUTIL_NXAST_AUTOPATH:
         naa = (const struct nx_action_autopath *)a;
         ds_put_format(s, "autopath(%u,", ntohl(naa->id));
-        nxm_format_field_bits(s, ntohl(naa->dst),
-                              nxm_decode_ofs(naa->ofs_nbits),
-                              nxm_decode_n_bits(naa->ofs_nbits));
+        nxm_decode(&subfield, naa->dst, naa->ofs_nbits);
+        mf_format_subfield(&subfield, s);
         ds_put_char(s, ')');
         break;
 
@@ -333,9 +334,8 @@ ofp_print_action(struct ds *s, const union ofp_action *a,
     case OFPUTIL_NXAST_OUTPUT_REG:
         naor = (const struct nx_action_output_reg *) a;
         ds_put_cstr(s, "output:");
-        nxm_format_field_bits(s, ntohl(naor->src),
-                              nxm_decode_ofs(naor->ofs_nbits),
-                              nxm_decode_n_bits(naor->ofs_nbits));
+        nxm_decode(&subfield, naor->src, naor->ofs_nbits);
+        mf_format_subfield(&subfield, s);
         break;
 
     case OFPUTIL_NXAST_LEARN:
diff --git a/lib/ofp-util.c b/lib/ofp-util.c
index 6fe1611e..2565ec7f 100644
--- a/lib/ofp-util.c
+++ b/lib/ofp-util.c
@@ -29,6 +29,7 @@
 #include "dynamic-string.h"
 #include "learn.h"
 #include "multipath.h"
+#include "meta-flow.h"
 #include "nx-match.h"
 #include "ofp-errors.h"
 #include "ofp-util.h"
@@ -2320,6 +2321,7 @@ static enum ofperr
 check_output_reg(const struct nx_action_output_reg *naor,
                  const struct flow *flow)
 {
+    struct mf_subfield src;
     size_t i;
 
     for (i = 0; i < sizeof naor->zero; i++) {
@@ -2328,8 +2330,8 @@ check_output_reg(const struct nx_action_output_reg *naor,
         }
     }
 
-    return nxm_src_check(naor->src, nxm_decode_ofs(naor->ofs_nbits),
-                         nxm_decode_n_bits(naor->ofs_nbits), flow);
+    nxm_decode(&src, naor->src, naor->ofs_nbits);
+    return mf_check_src(&src, flow);
 }
 
 enum ofperr
diff --git a/ofproto/ofproto-dpif.c b/ofproto/ofproto-dpif.c
index 53d3c9fe..a0c7b288 100644
--- a/ofproto/ofproto-dpif.c
+++ b/ofproto/ofproto-dpif.c
@@ -34,6 +34,7 @@
 #include "lacp.h"
 #include "learn.h"
 #include "mac-learning.h"
+#include "meta-flow.h"
 #include "multipath.h"
 #include "netdev.h"
 #include "netlink.h"
@@ -4568,9 +4569,11 @@ static void
 xlate_output_reg_action(struct action_xlate_ctx *ctx,
                         const struct nx_action_output_reg *naor)
 {
+    struct mf_subfield src;
     uint64_t ofp_port;
 
-    ofp_port = nxm_read_field_bits(naor->src, naor->ofs_nbits, &ctx->flow);
+    nxm_decode(&src, naor->src, naor->ofs_nbits);
+    ofp_port = mf_get_subfield(&src, &ctx->flow);
 
     if (ofp_port <= UINT16_MAX) {
         xlate_output_action__(ctx, ofp_port, ntohs(naor->max_len));