It is possible to do this translation without adding a new data
structure, by modifying the code in @file{userprog/pagedir.c}. However,
-if you do that you'll need to carefully study and understand section 3.7
-in @bibref{IA32-v3}, and in practice it is probably easier to add a new
+if you do that you'll need to carefully study and understand section
+3.7, ``Page Translation Using 32-Bit Physical Addressing,'' in
+@bibref{IA32-v3a}, and in practice it is probably easier to add a new
data structure.
@item
Allocate additional pages only if they ``appear'' to be stack accesses.
Devise a heuristic that attempts to distinguish stack accesses from
-other accesses. You can retrieve the user program's current stack
-pointer from the @struct{intr_frame}'s @code{esp} member.
+other accesses.
User programs are buggy if they write to the stack below the stack
pointer, because typical real OSes may interrupt a process at any time
@code{PUSHA} instruction pushes 32 bytes at once, so it can fault 32
bytes below the stack pointer.
+You will need to be able to obtain the current value of the user
+program's stack pointer. Within a system call or a page fault generated
+by a user program, you can retrieve it from @code{esp} member of the
+@struct{intr_frame} passed to @func{syscall_handler} or
+@func{page_fault}, respectively. If you verify user pointers before
+accessing them (@pxref{Accessing User Memory}), these are the only cases
+you need to handle. On the other hand, if you depend on page faults to
+detect invalid memory access, you will need to handle another case,
+where a page fault occurs in the kernel. Reading @code{esp} out of the
+@struct{intr_frame} passed to @func{page_fault} in that case will obtain
+the kernel stack pointer, not the user stack pointer. You will need to
+arrange another way, e.g.@: by saving @code{esp} into @struct{thread} on
+the initial transition from user to kernel mode.
+
You may impose some absolute limit on stack size, as do most OSes.
-(Some OSes make the limit user-adjustable, e.g.@: with the
-@command{ulimit} command on many Unix systems.)
+Some OSes make the limit user-adjustable, e.g.@: with the
+@command{ulimit} command on many Unix systems. On many GNU/Linux systems,
+the default limit is 8 MB.
The first stack page need not be allocated lazily. You can initialize
it with the command line arguments at load time, with no need to wait
17 files changed, 1532 insertions(+), 104 deletions(-)
@end verbatim
-@item Do we need a working HW 2 to implement HW 3?
+@item Do we need a working Project 2 to implement Project 3?
Yes.
You can layer some other allocator on top of @func{palloc_get_page} if
you like, but it should be the underlying mechanism.
-Also, you can use the @option{-u} option to @command{pintos} to limit
+Also, you can use the @option{-ul} option to @command{pintos} to limit
the size of the user pool, which makes it easy to test your VM
implementation with various user memory sizes.
it like so:
@example
-write (addr, 64, STDOUT_FILENO);
+write (STDOUT_FILENO, addr, 64);
@end example
Similarly, if you wanted to replace the first byte of the file,