[PATCH] i386: Use %gs as the PDA base-segment in the kernel

This patch is the meat of the PDA change.  This patch makes several related
changes:

1: Most significantly, %gs is now used in the kernel.  This means that on
   entry, the old value of %gs is saved away, and it is reloaded with
   __KERNEL_PDA.

2: entry.S constructs the stack in the shape of struct pt_regs, and this
   is passed around the kernel so that the process's saved register
   state can be accessed.

   Unfortunately struct pt_regs doesn't currently have space for %gs
   (or %fs). This patch extends pt_regs to add space for gs (no space
   is allocated for %fs, since it won't be used, and it would just
   complicate the code in entry.S to work around the space).

3: Because %gs is now saved on the stack like %ds, %es and the integer
   registers, there are a number of places where it no longer needs to
   be handled specially; namely context switch, and saving/restoring the
   register state in a signal context.

4: And since kernel threads run in kernel space and call normal kernel
   code, they need to be created with their %gs == __KERNEL_PDA.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Chuck Ebbert <76306.1226@compuserve.com>
Cc: Zachary Amsden <zach@vmware.com>
Cc: Jan Beulich <jbeulich@novell.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>

1 files changed, 48 insertions, 22 deletions

diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 0069bf01603e..b99d4a160078 100644
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -30,12 +30,13 @@
  *      18(%esp) - %eax
  *      1C(%esp) - %ds
  *      20(%esp) - %es
- *      24(%esp) - orig_eax
- *      28(%esp) - %eip
- *      2C(%esp) - %cs
- *      30(%esp) - %eflags
- *      34(%esp) - %oldesp
- *      38(%esp) - %oldss
+ *      24(%esp) - %gs
+ *      28(%esp) - orig_eax
+ *      2C(%esp) - %eip
+ *      30(%esp) - %cs
+ *      34(%esp) - %eflags
+ *      38(%esp) - %oldesp
+ *      3C(%esp) - %oldss
  *
  * "current" is in register %ebx during any slow entries.
  */
@@ -92,6 +93,9 @@ VM_MASK		= 0x00020000
 
 #define SAVE_ALL \
         cld; \
+        pushl %gs; \
+        CFI_ADJUST_CFA_OFFSET 4;\
+        /*CFI_REL_OFFSET gs, 0;*/\
         pushl %es; \
         CFI_ADJUST_CFA_OFFSET 4;\
         /*CFI_REL_OFFSET es, 0;*/\
@@ -121,7 +125,9 @@ VM_MASK		= 0x00020000
         CFI_REL_OFFSET ebx, 0;\
         movl $(__USER_DS), %edx; \
         movl %edx, %ds; \
-        movl %edx, %es;
+        movl %edx, %es; \
+        movl $(__KERNEL_PDA), %edx; \
+        movl %edx, %gs
 
 #define RESTORE_INT_REGS \
         popl %ebx;      \
@@ -154,17 +160,22 @@ VM_MASK		= 0x00020000
 2:      popl %es;       \
         CFI_ADJUST_CFA_OFFSET -4;\
         /*CFI_RESTORE es;*/\
-.section .fixup,"ax";   \
-3:      movl $0,(%esp); \
-        jmp 1b;         \
+3:      popl %gs;       \
+        CFI_ADJUST_CFA_OFFSET -4;\
+        /*CFI_RESTORE gs;*/\
+.pushsection .fixup,"ax";       \
 4:      movl $0,(%esp); \
+        jmp 1b;         \
+5:      movl $0,(%esp); \
         jmp 2b;         \
-.previous;              \
+6:      movl $0,(%esp); \
+        jmp 3b;         \
 .section __ex_table,"a";\
         .align 4;       \
-        .long 1b,3b;    \
-        .long 2b,4b;    \
-.previous
+        .long 1b,4b;    \
+        .long 2b,5b;    \
+        .long 3b,6b;    \
+.popsection
 
 #define RING0_INT_FRAME \
         CFI_STARTPROC simple;\
@@ -231,6 +242,7 @@ check_userspace:
         andl $(VM_MASK | SEGMENT_RPL_MASK), %eax
         cmpl $USER_RPL, %eax
         jb resume_kernel                # not returning to v8086 or userspace
+
 ENTRY(resume_userspace)
         DISABLE_INTERRUPTS              # make sure we don't miss an interrupt
                                         # setting need_resched or sigpending
@@ -327,9 +339,16 @@ sysenter_past_esp:
         movl PT_OLDESP(%esp), %ecx
         xorl %ebp,%ebp
         TRACE_IRQS_ON
+1:      mov  PT_GS(%esp), %gs
         ENABLE_INTERRUPTS_SYSEXIT
         CFI_ENDPROC
-
+.pushsection .fixup,"ax"
+2:      movl $0,PT_GS(%esp)
+        jmp 1b
+.section __ex_table,"a"
+        .align 4
+        .long 1b,2b
+.popsection
 
         # system call handler stub
 ENTRY(system_call)
@@ -375,7 +394,7 @@ restore_nocheck:
         TRACE_IRQS_IRET
 restore_nocheck_notrace:
         RESTORE_REGS
-        addl $4, %esp
+        addl $4, %esp                   # skip orig_eax/error_code
         CFI_ADJUST_CFA_OFFSET -4
 1:      INTERRUPT_RETURN
 .section .fixup,"ax"
@@ -588,6 +607,10 @@ KPROBE_ENTRY(page_fault)
         CFI_ADJUST_CFA_OFFSET 4
         ALIGN
 error_code:
+        /* the function address is in %gs's slot on the stack */
+        pushl %es
+        CFI_ADJUST_CFA_OFFSET 4
+        /*CFI_REL_OFFSET es, 0*/
         pushl %ds
         CFI_ADJUST_CFA_OFFSET 4
         /*CFI_REL_OFFSET ds, 0*/
@@ -613,18 +636,20 @@ error_code:
         CFI_ADJUST_CFA_OFFSET 4
         CFI_REL_OFFSET ebx, 0
         cld
-        pushl %es
+        pushl %gs
         CFI_ADJUST_CFA_OFFSET 4
-        /*CFI_REL_OFFSET es, 0*/
+        /*CFI_REL_OFFSET gs, 0*/
+        movl $(__KERNEL_PDA), %ecx
+        movl %ecx, %gs
         UNWIND_ESPFIX_STACK
         popl %ecx
         CFI_ADJUST_CFA_OFFSET -4
         /*CFI_REGISTER es, ecx*/
-        movl PT_ES(%esp), %edi          # get the function address
+        movl PT_GS(%esp), %edi          # get the function address
         movl PT_ORIG_EAX(%esp), %edx    # get the error code
-        movl $-1, PT_ORIG_EAX(%esp)
-        movl %ecx, PT_ES(%esp)
-        /*CFI_REL_OFFSET es, ES*/
+        movl $-1, PT_ORIG_EAX(%esp)     # no syscall to restart
+        mov  %ecx, PT_GS(%esp)
+        /*CFI_REL_OFFSET gs, ES*/
         movl $(__USER_DS), %ecx
         movl %ecx, %ds
         movl %ecx, %es
@@ -936,6 +961,7 @@ ENTRY(arch_unwind_init_running)
         movl    %ebx, PT_EAX(%edx)
         movl    $__USER_DS, PT_DS(%edx)
         movl    $__USER_DS, PT_ES(%edx)
+        movl    $0, PT_GS(%edx)
         movl    %ebx, PT_ORIG_EAX(%edx)
         movl    %ecx, PT_EIP(%edx)
         movl    12(%esp), %ecx