[PATCH] i386: Convert i386 PDA code to use %fs

Convert the PDA code to use %fs rather than %gs as the segment for
per-processor data.  This is because some processors show a small but
measurable performance gain for reloading a NULL segment selector (as %fs
generally is in user-space) versus a non-NULL one (as %gs generally is).

On modern processors the difference is very small, perhaps undetectable.
Some old AMD "K6 3D+" processors are noticably slower when %fs is used
rather than %gs; I have no idea why this might be, but I think they're
sufficiently rare that it doesn't matter much.

This patch also fixes the math emulator, which had not been adjusted to
match the changed struct pt_regs.

[frederik.deweerdt@gmail.com: fixit with gdb]
[mingo@elte.hu: Fix KVM too]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Ian Campbell <Ian.Campbell@XenSource.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Zachary Amsden <zach@vmware.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>

1 files changed, 5 insertions, 9 deletions

diff --git a/arch/i386/math-emu/get_address.c b/arch/i386/math-emu/get_address.c
index 9819b705efa4..2e2c51a8bd3a 100644
--- a/arch/i386/math-emu/get_address.c
+++ b/arch/i386/math-emu/get_address.c
@@ -56,15 +56,14 @@ static int reg_offset_vm86[] = {
 #define VM86_REG_(x) (*(unsigned short *) \
                       (reg_offset_vm86[((unsigned)x)]+(u_char *) FPU_info))
 
-/* These are dummy, fs and gs are not saved on the stack. */
-#define ___FS ___ds
+/* This dummy, gs is not saved on the stack. */
 #define ___GS ___ds
 
 static int reg_offset_pm[] = {
         offsetof(struct info,___cs),
         offsetof(struct info,___ds),
         offsetof(struct info,___es),
-        offsetof(struct info,___FS),
+        offsetof(struct info,___fs),
         offsetof(struct info,___GS),
         offsetof(struct info,___ss),
         offsetof(struct info,___ds)
@@ -169,13 +168,10 @@ static long pm_address(u_char FPU_modrm, u_char segment,
 
   switch ( segment )
     {
-      /* fs and gs aren't used by the kernel, so they still have their
-         user-space values. */
-    case PREFIX_FS_-1:
-      /* N.B. - movl %seg, mem is a 2 byte write regardless of prefix */
-      savesegment(fs, addr->selector);
-      break;
+      /* gs isn't used by the kernel, so it still has its
+         user-space value. */
     case PREFIX_GS_-1:
+      /* N.B. - movl %seg, mem is a 2 byte write regardless of prefix */
       savesegment(gs, addr->selector);
       break;
     default: