--- /dev/null
+#include "gdt.h"
+#include "debug.h"
+#include "mmu.h"
+#include "palloc.h"
+#include "tss.h"
+
+/* The Global Descriptor Table (GDT).
+
+ The GDT, an x86-specific structure, defines segments that can
+ potentially be used by all processes in a system, subject to
+ their permissions. There is also a per-process Local
+ Descriptor Table (LDT) but that is not used by modern
+ operating systems.
+
+ Each entry in the GDT, which is known by its byte offset in
+ the table, identifies a segment. For our purposes only three
+ types of segments are of interest: code, data, and TSS or
+ Task-State Segment descriptors. The former two types are
+ exactly what they sound like. The TSS is used primarily for
+ stack switching on interrupts.
+
+ For more information on the GDT as used here, refer to
+ [IA32-v3] sections 3.2 through 3.5. */
+static uint64_t gdt[SEL_CNT];
+
+/* GDT helpers. */
+static uint64_t make_code_desc (int dpl);
+static uint64_t make_data_desc (int dpl);
+static uint64_t make_tss_desc (void *laddr);
+static uint64_t make_gdtr_operand (uint16_t limit, void *base);
+
+/* Sets up a proper GDT. The bootstrap loader's GDT didn't
+ include user-mode selectors or a TSS, but we need both now. */
+void
+gdt_init (void)
+{
+ uint64_t gdtr_operand;
+
+ /* Initialize GDT. */
+ gdt[SEL_NULL / sizeof *gdt] = 0;
+ gdt[SEL_KCSEG / sizeof *gdt] = make_code_desc (0);
+ gdt[SEL_KDSEG / sizeof *gdt] = make_data_desc (0);
+ gdt[SEL_UCSEG / sizeof *gdt] = make_code_desc (3);
+ gdt[SEL_UDSEG / sizeof *gdt] = make_data_desc (3);
+ gdt[SEL_TSS / sizeof *gdt] = make_tss_desc (tss_get ());
+
+ /* Load GDTR, TR. */
+ gdtr_operand = make_gdtr_operand (sizeof gdt - 1, gdt);
+ asm volatile ("lgdt %0" :: "m" (gdtr_operand));
+ asm volatile ("ltr %w0" :: "r" (SEL_TSS));
+}
+\f
+/* System segment or code/data segment? */
+enum seg_class
+ {
+ CLS_SYSTEM = 0, /* System segment. */
+ CLS_CODE_DATA = 1 /* Code or data segment. */
+ };
+
+/* Limit has byte or 4 kB page granularity? */
+enum seg_granularity
+ {
+ GRAN_BYTE = 0, /* Limit has 1-byte granularity. */
+ GRAN_PAGE = 1 /* Limit has 4 kB granularity. */
+ };
+
+/* Returns a segment descriptor with the given 32-bit BASE and
+ 20-bit LIMIT (whose interpretation depends on GRANULARITY).
+ The descriptor represents a system or code/data segment
+ according to CLASS, and TYPE is its type (whose interpretation
+ depends on the class).
+
+ The segment has descriptor privilege level DPL, meaning that
+ it can be used in rings numbered DPL or lower. In practice,
+ DPL==3 means that user processes can use the segment and
+ DPL==0 means that only the kernel can use the segment. See
+ [IA32-v3] section 4.5 for further discussion. */
+static uint64_t
+make_seg_desc (uint32_t base,
+ uint32_t limit,
+ enum seg_class class,
+ int type,
+ int dpl,
+ enum seg_granularity granularity)
+{
+ uint32_t e0, e1;
+
+ ASSERT (limit <= 0xfffff);
+ ASSERT (class == CLS_SYSTEM || class == CLS_CODE_DATA);
+ ASSERT (type >= 0 && type <= 15);
+ ASSERT (dpl >= 0 && dpl <= 3);
+ ASSERT (granularity == GRAN_BYTE || granularity == GRAN_PAGE);
+
+ e0 = ((limit & 0xffff) /* Limit 15:0. */
+ | (base << 16)); /* Base 15:0. */
+
+ e1 = (((base >> 16) & 0xff) /* Base 23:16. */
+ | (type << 8) /* Segment type. */
+ | (class << 12) /* 0=system, 1=code/data. */
+ | (dpl << 13) /* Descriptor privilege. */
+ | (1 << 15) /* Present. */
+ | (limit & 0xf0000) /* Limit 16:19. */
+ | (1 << 22) /* 32-bit segment. */
+ | (granularity << 23) /* Byte/page granularity. */
+ | (base & 0xff000000)); /* Base 31:24. */
+
+ return e0 | ((uint64_t) e1 << 32);
+}
+
+/* Returns a descriptor for a readable code segment with base at
+ 0, a limit of 4 GB, and the given DPL. */
+static uint64_t
+make_code_desc (int dpl)
+{
+ return make_seg_desc (0, 0xfffff, CLS_CODE_DATA, 10, dpl, GRAN_PAGE);
+}
+
+/* Returns a descriptor for a writable data segment with base at
+ 0, a limit of 4 GB, and the given DPL. */
+static uint64_t
+make_data_desc (int dpl)
+{
+ return make_seg_desc (0, 0xfffff, CLS_CODE_DATA, 2, dpl, GRAN_PAGE);
+}
+
+/* Returns a descriptor for an "available" 32-bit Task-State
+ Segment with its base at the given linear address, a limit of
+ 0x67 bytes (the size of a 32-bit TSS), and a DPL of 0. */
+static uint64_t
+make_tss_desc (void *laddr)
+{
+ return make_seg_desc ((uint32_t) laddr, 0x67, CLS_SYSTEM, 9, 0, GRAN_BYTE);
+}
+
+
+/* Returns a descriptor that yields the given LIMIT and BASE when
+ used as an operand for the LGDT instruction. */
+static uint64_t
+make_gdtr_operand (uint16_t limit, void *base)
+{
+ return limit | ((uint64_t) (uint32_t) base << 16);
+}