[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, 11 insertions, 13 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index c641056233a6..23ae198dbbc3 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -308,8 +308,8 @@ void show_regs(struct pt_regs * regs)
                 regs->eax,regs->ebx,regs->ecx,regs->edx);
         printk("ESI: %08lx EDI: %08lx EBP: %08lx",
                 regs->esi, regs->edi, regs->ebp);
-        printk(" DS: %04x ES: %04x GS: %04x\n",
-               0xffff & regs->xds,0xffff & regs->xes, 0xffff & regs->xgs);
+        printk(" DS: %04x ES: %04x FS: %04x\n",
+               0xffff & regs->xds,0xffff & regs->xes, 0xffff & regs->xfs);
 
         cr0 = read_cr0();
         cr2 = read_cr2();
@@ -340,7 +340,7 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 
         regs.xds = __USER_DS;
         regs.xes = __USER_DS;
-        regs.xgs = __KERNEL_PDA;
+        regs.xfs = __KERNEL_PDA;
         regs.orig_eax = -1;
         regs.eip = (unsigned long) kernel_thread_helper;
         regs.xcs = __KERNEL_CS | get_kernel_rpl();
@@ -425,7 +425,7 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
 
         p->thread.eip = (unsigned long) ret_from_fork;
 
-        savesegment(fs,p->thread.fs);
+        savesegment(gs,p->thread.gs);
 
         tsk = current;
         if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
@@ -501,8 +501,8 @@ void dump_thread(struct pt_regs * regs, struct user * dump)
         dump->regs.eax = regs->eax;
         dump->regs.ds = regs->xds;
         dump->regs.es = regs->xes;
-        savesegment(fs,dump->regs.fs);
-        dump->regs.gs = regs->xgs;
+        dump->regs.fs = regs->xfs;
+        savesegment(gs,dump->regs.gs);
         dump->regs.orig_eax = regs->orig_eax;
         dump->regs.eip = regs->eip;
         dump->regs.cs = regs->xcs;
@@ -653,7 +653,7 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
         load_esp0(tss, next);
 
         /*
-         * Save away %fs. No need to save %gs, as it was saved on the
+         * Save away %gs. No need to save %fs, as it was saved on the
          * stack on entry.  No need to save %es and %ds, as those are
          * always kernel segments while inside the kernel.  Doing this
          * before setting the new TLS descriptors avoids the situation
@@ -662,7 +662,7 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
          * used %fs or %gs (it does not today), or if the kernel is
          * running inside of a hypervisor layer.
          */
-        savesegment(fs, prev->fs);
+        savesegment(gs, prev->gs);
 
         /*
          * Load the per-thread Thread-Local Storage descriptor.
@@ -670,12 +670,10 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
         load_TLS(next, cpu);
 
         /*
-         * Restore %fs if needed.
-         *
-         * Glibc normally makes %fs be zero.
+         * Restore %gs if needed (which is common)
          */
-        if (unlikely(prev->fs | next->fs))
-                loadsegment(fs, next->fs);
+        if (prev->gs | next->gs)
+                loadsegment(gs, next->gs);
 
         write_pda(pcurrent, next_p);