[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>

11 files changed, 79 insertions, 83 deletions

diff --git a/arch/i386/kernel/asm-offsets.c b/arch/i386/kernel/asm-offsets.c
index 1b2f3cd33270..c37535163bfc 100644
--- a/arch/i386/kernel/asm-offsets.c
+++ b/arch/i386/kernel/asm-offsets.c
@@ -72,7 +72,7 @@ void foo(void)
         OFFSET(PT_EAX, pt_regs, eax);
         OFFSET(PT_DS,  pt_regs, xds);
         OFFSET(PT_ES,  pt_regs, xes);
-        OFFSET(PT_GS,  pt_regs, xgs);
+        OFFSET(PT_FS,  pt_regs, xfs);
         OFFSET(PT_ORIG_EAX, pt_regs, orig_eax);
         OFFSET(PT_EIP, pt_regs, eip);
         OFFSET(PT_CS,  pt_regs, xcs);
diff --git a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
index 8a8bbdaaf38a..dcbbd0a8bfc2 100644
--- a/arch/i386/kernel/cpu/common.c
+++ b/arch/i386/kernel/cpu/common.c
@@ -605,7 +605,7 @@ void __init early_cpu_init(void)
 struct pt_regs * __devinit idle_regs(struct pt_regs *regs)
 {
         memset(regs, 0, sizeof(struct pt_regs));
-        regs->xgs = __KERNEL_PDA;
+        regs->xfs = __KERNEL_PDA;
         return regs;
 }
 
@@ -662,12 +662,12 @@ struct i386_pda boot_pda = {
         .pcurrent = &init_task,
 };
 
-static inline void set_kernel_gs(void)
+static inline void set_kernel_fs(void)
 {
-        /* Set %gs for this CPU's PDA.  Memory clobber is to create a
+        /* Set %fs for this CPU's PDA.  Memory clobber is to create a
            barrier with respect to any PDA operations, so the compiler
            doesn't move any before here. */
-        asm volatile ("mov %0, %%gs" : : "r" (__KERNEL_PDA) : "memory");
+        asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_PDA) : "memory");
 }
 
 /* Initialize the CPU's GDT and PDA.  The boot CPU does this for
@@ -718,7 +718,7 @@ void __cpuinit cpu_set_gdt(int cpu)
            the boot CPU, this will transition from the boot gdt+pda to
            the real ones). */
         load_gdt(cpu_gdt_descr);
-        set_kernel_gs();
+        set_kernel_fs();
 }
 
 /* Common CPU init for both boot and secondary CPUs */
@@ -764,8 +764,8 @@ static void __cpuinit _cpu_init(int cpu, struct task_struct *curr)
         __set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS, &doublefault_tss);
 #endif
 
-        /* Clear %fs. */
-        asm volatile ("mov %0, %%fs" : : "r" (0));
+        /* Clear %gs. */
+        asm volatile ("mov %0, %%gs" : : "r" (0));
 
         /* Clear all 6 debug registers: */
         set_debugreg(0, 0);
diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 5e47683fc63a..8c6a22a42d2e 100644
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -30,7 +30,7 @@
  *      18(%esp) - %eax
  *      1C(%esp) - %ds
  *      20(%esp) - %es
- *      24(%esp) - %gs
+ *      24(%esp) - %fs
  *      28(%esp) - orig_eax
  *      2C(%esp) - %eip
  *      30(%esp) - %cs
@@ -99,9 +99,9 @@ VM_MASK		= 0x00020000
 
 #define SAVE_ALL \
         cld; \
-        pushl %gs; \
+        pushl %fs; \
         CFI_ADJUST_CFA_OFFSET 4;\
-        /*CFI_REL_OFFSET gs, 0;*/\
+        /*CFI_REL_OFFSET fs, 0;*/\
         pushl %es; \
         CFI_ADJUST_CFA_OFFSET 4;\
         /*CFI_REL_OFFSET es, 0;*/\
@@ -133,7 +133,7 @@ VM_MASK		= 0x00020000
         movl %edx, %ds; \
         movl %edx, %es; \
         movl $(__KERNEL_PDA), %edx; \
-        movl %edx, %gs
+        movl %edx, %fs
 
 #define RESTORE_INT_REGS \
         popl %ebx;      \
@@ -166,9 +166,9 @@ VM_MASK		= 0x00020000
 2:      popl %es;       \
         CFI_ADJUST_CFA_OFFSET -4;\
         /*CFI_RESTORE es;*/\
-3:      popl %gs;       \
+3:      popl %fs;       \
         CFI_ADJUST_CFA_OFFSET -4;\
-        /*CFI_RESTORE gs;*/\
+        /*CFI_RESTORE fs;*/\
 .pushsection .fixup,"ax";       \
 4:      movl $0,(%esp); \
         jmp 1b;         \
@@ -349,11 +349,11 @@ sysenter_past_esp:
         movl PT_OLDESP(%esp), %ecx
         xorl %ebp,%ebp
         TRACE_IRQS_ON
-1:      mov  PT_GS(%esp), %gs
+1:      mov  PT_FS(%esp), %fs
         ENABLE_INTERRUPTS_SYSEXIT
         CFI_ENDPROC
 .pushsection .fixup,"ax"
-2:      movl $0,PT_GS(%esp)
+2:      movl $0,PT_FS(%esp)
         jmp 1b
 .section __ex_table,"a"
         .align 4
@@ -550,7 +550,7 @@ syscall_badsys:
 
 #define FIXUP_ESPFIX_STACK \
         /* since we are on a wrong stack, we cant make it a C code :( */ \
-        movl %gs:PDA_cpu, %ebx; \
+        movl %fs:PDA_cpu, %ebx; \
         PER_CPU(cpu_gdt_descr, %ebx); \
         movl GDS_address(%ebx), %ebx; \
         GET_DESC_BASE(GDT_ENTRY_ESPFIX_SS, %ebx, %eax, %ax, %al, %ah); \
@@ -632,7 +632,7 @@ KPROBE_ENTRY(page_fault)
         CFI_ADJUST_CFA_OFFSET 4
         ALIGN
 error_code:
-        /* the function address is in %gs's slot on the stack */
+        /* the function address is in %fs's slot on the stack */
         pushl %es
         CFI_ADJUST_CFA_OFFSET 4
         /*CFI_REL_OFFSET es, 0*/
@@ -661,20 +661,20 @@ error_code:
         CFI_ADJUST_CFA_OFFSET 4
         CFI_REL_OFFSET ebx, 0
         cld
-        pushl %gs
+        pushl %fs
         CFI_ADJUST_CFA_OFFSET 4
-        /*CFI_REL_OFFSET gs, 0*/
+        /*CFI_REL_OFFSET fs, 0*/
         movl $(__KERNEL_PDA), %ecx
-        movl %ecx, %gs
+        movl %ecx, %fs
         UNWIND_ESPFIX_STACK
         popl %ecx
         CFI_ADJUST_CFA_OFFSET -4
         /*CFI_REGISTER es, ecx*/
-        movl PT_GS(%esp), %edi          # get the function address
+        movl PT_FS(%esp), %edi          # get the function address
         movl PT_ORIG_EAX(%esp), %edx    # get the error code
         movl $-1, PT_ORIG_EAX(%esp)     # no syscall to restart
-        mov  %ecx, PT_GS(%esp)
-        /*CFI_REL_OFFSET gs, ES*/
+        mov  %ecx, PT_FS(%esp)
+        /*CFI_REL_OFFSET fs, ES*/
         movl $(__USER_DS), %ecx
         movl %ecx, %ds
         movl %ecx, %es
diff --git a/arch/i386/kernel/head.S b/arch/i386/kernel/head.S
index cb9abdfced9b..15336c8b5960 100644
--- a/arch/i386/kernel/head.S
+++ b/arch/i386/kernel/head.S
@@ -319,12 +319,12 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%ds
         movl %eax,%es
 
-        xorl %eax,%eax                  # Clear FS and LDT
-        movl %eax,%fs
+        xorl %eax,%eax                  # Clear GS and LDT
+        movl %eax,%gs
         lldt %ax
 
         movl $(__KERNEL_PDA),%eax
-        mov  %eax,%gs
+        mov  %eax,%fs
 
         cld                     # gcc2 wants the direction flag cleared at all times
         pushl $0                # fake return address for unwinder
diff --git a/arch/i386/kernel/kprobes.c b/arch/i386/kernel/kprobes.c
index af1d53344993..b85cfa3ce1dd 100644
--- a/arch/i386/kernel/kprobes.c
+++ b/arch/i386/kernel/kprobes.c
@@ -363,7 +363,7 @@ no_kprobe:
                         "       pushf\n"
                         /* skip cs, eip, orig_eax */
                         "       subl $12, %esp\n"
-                        "       pushl %gs\n"
+                        "       pushl %fs\n"
                         "       pushl %ds\n"
                         "       pushl %es\n"
                         "       pushl %eax\n"
@@ -387,7 +387,7 @@ no_kprobe:
                         "       popl %edi\n"
                         "       popl %ebp\n"
                         "       popl %eax\n"
-                        /* skip eip, orig_eax, es, ds, gs */
+                        /* skip eip, orig_eax, es, ds, fs */
                         "       addl $20, %esp\n"
                         "       popf\n"
                         "       ret\n");
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);
 
diff --git a/arch/i386/kernel/ptrace.c b/arch/i386/kernel/ptrace.c
index af8aabe85800..4a8f8a259723 100644
--- a/arch/i386/kernel/ptrace.c
+++ b/arch/i386/kernel/ptrace.c
@@ -89,14 +89,14 @@ static int putreg(struct task_struct *child,
         unsigned long regno, unsigned long value)
 {
         switch (regno >> 2) {
-                case FS:
+                case GS:
                         if (value && (value & 3) != 3)
                                 return -EIO;
-                        child->thread.fs = value;
+                        child->thread.gs = value;
                         return 0;
                 case DS:
                 case ES:
-                case GS:
+                case FS:
                         if (value && (value & 3) != 3)
                                 return -EIO;
                         value &= 0xffff;
@@ -112,7 +112,7 @@ static int putreg(struct task_struct *child,
                         value |= get_stack_long(child, EFL_OFFSET) & ~FLAG_MASK;
                         break;
         }
-        if (regno > ES*4)
+        if (regno > FS*4)
                 regno -= 1*4;
         put_stack_long(child, regno, value);
         return 0;
@@ -124,18 +124,18 @@ static unsigned long getreg(struct task_struct *child,
         unsigned long retval = ~0UL;
 
         switch (regno >> 2) {
-                case FS:
-                        retval = child->thread.fs;
+                case GS:
+                        retval = child->thread.gs;
                         break;
                 case DS:
                 case ES:
-                case GS:
+                case FS:
                 case SS:
                 case CS:
                         retval = 0xffff;
                         /* fall through */
                 default:
-                        if (regno > ES*4)
+                        if (regno > FS*4)
                                 regno -= 1*4;
                         retval &= get_stack_long(child, regno);
         }
diff --git a/arch/i386/kernel/signal.c b/arch/i386/kernel/signal.c
index 65d7620eaa09..8f4afcc7d2ab 100644
--- a/arch/i386/kernel/signal.c
+++ b/arch/i386/kernel/signal.c
@@ -128,8 +128,8 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, int *peax
                          X86_EFLAGS_TF | X86_EFLAGS_SF | X86_EFLAGS_ZF | \
                          X86_EFLAGS_AF | X86_EFLAGS_PF | X86_EFLAGS_CF)
 
-        COPY_SEG(gs);
-        GET_SEG(fs);
+        GET_SEG(gs);
+        COPY_SEG(fs);
         COPY_SEG(es);
         COPY_SEG(ds);
         COPY(edi);
@@ -244,9 +244,9 @@ setup_sigcontext(struct sigcontext __user *sc, struct _fpstate __user *fpstate,
 {
         int tmp, err = 0;
 
-        err |= __put_user(regs->xgs, (unsigned int __user *)&sc->gs);
-        savesegment(fs, tmp);
-        err |= __put_user(tmp, (unsigned int __user *)&sc->fs);
+        err |= __put_user(regs->xfs, (unsigned int __user *)&sc->fs);
+        savesegment(gs, tmp);
+        err |= __put_user(tmp, (unsigned int __user *)&sc->gs);
 
         err |= __put_user(regs->xes, (unsigned int __user *)&sc->es);
         err |= __put_user(regs->xds, (unsigned int __user *)&sc->ds);
diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c
index 0efad8aeb41a..4ec21037a361 100644
--- a/arch/i386/kernel/traps.c
+++ b/arch/i386/kernel/traps.c
@@ -291,10 +291,11 @@ void show_registers(struct pt_regs *regs)
         int i;
         int in_kernel = 1;
         unsigned long esp;
-        unsigned short ss;
+        unsigned short ss, gs;
 
         esp = (unsigned long) (&regs->esp);
         savesegment(ss, ss);
+        savesegment(gs, gs);
         if (user_mode_vm(regs)) {
                 in_kernel = 0;
                 esp = regs->esp;
@@ -313,8 +314,8 @@ void show_registers(struct pt_regs *regs)
                 regs->eax, regs->ebx, regs->ecx, regs->edx);
         printk(KERN_EMERG "esi: %08lx   edi: %08lx   ebp: %08lx   esp: %08lx\n",
                 regs->esi, regs->edi, regs->ebp, esp);
-        printk(KERN_EMERG "ds: %04x   es: %04x   ss: %04x\n",
-                regs->xds & 0xffff, regs->xes & 0xffff, ss);
+        printk(KERN_EMERG "ds: %04x   es: %04x   fs: %04x  gs: %04x  ss: %04x\n",
+               regs->xds & 0xffff, regs->xes & 0xffff, regs->xfs & 0xffff, gs, ss);
         printk(KERN_EMERG "Process %.*s (pid: %d, ti=%p task=%p task.ti=%p)",
                 TASK_COMM_LEN, current->comm, current->pid,
                 current_thread_info(), current, current->thread_info);
diff --git a/arch/i386/kernel/vm86.c b/arch/i386/kernel/vm86.c
index be2f96e67f78..d1b8f2b7aea6 100644
--- a/arch/i386/kernel/vm86.c
+++ b/arch/i386/kernel/vm86.c
@@ -96,12 +96,12 @@ static int copy_vm86_regs_to_user(struct vm86_regs __user *user,
 {
         int ret = 0;
 
-        /* kernel_vm86_regs is missing xfs, so copy everything up to
-           (but not including) xgs, and then rest after xgs. */
-        ret += copy_to_user(user, regs, offsetof(struct kernel_vm86_regs, pt.xgs));
-        ret += copy_to_user(&user->__null_gs, &regs->pt.xgs,
+        /* kernel_vm86_regs is missing xgs, so copy everything up to
+           (but not including) orig_eax, and then rest including orig_eax. */
+        ret += copy_to_user(user, regs, offsetof(struct kernel_vm86_regs, pt.orig_eax));
+        ret += copy_to_user(&user->orig_eax, &regs->pt.orig_eax,
                             sizeof(struct kernel_vm86_regs) -
-                            offsetof(struct kernel_vm86_regs, pt.xgs));
+                            offsetof(struct kernel_vm86_regs, pt.orig_eax));
 
         return ret;
 }
@@ -113,12 +113,13 @@ static int copy_vm86_regs_from_user(struct kernel_vm86_regs *regs,
 {
         int ret = 0;
 
-        ret += copy_from_user(regs, user, offsetof(struct kernel_vm86_regs, pt.xgs));
-        ret += copy_from_user(&regs->pt.xgs, &user->__null_gs,
+        /* copy eax-xfs inclusive */
+        ret += copy_from_user(regs, user, offsetof(struct kernel_vm86_regs, pt.orig_eax));
+        /* copy orig_eax-__gsh+extra */
+        ret += copy_from_user(&regs->pt.orig_eax, &user->orig_eax,
                               sizeof(struct kernel_vm86_regs) -
-                              offsetof(struct kernel_vm86_regs, pt.xgs) +
+                              offsetof(struct kernel_vm86_regs, pt.orig_eax) +
                               extra);
-
         return ret;
 }
 
@@ -157,8 +158,8 @@ struct pt_regs * fastcall save_v86_state(struct kernel_vm86_regs * regs)
 
         ret = KVM86->regs32;
 
-        loadsegment(fs, current->thread.saved_fs);
-        ret->xgs = current->thread.saved_gs;
+        ret->xfs = current->thread.saved_fs;
+        loadsegment(gs, current->thread.saved_gs);
 
         return ret;
 }
@@ -285,9 +286,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
  */
         info->regs.pt.xds = 0;
         info->regs.pt.xes = 0;
-        info->regs.pt.xgs = 0;
+        info->regs.pt.xfs = 0;
 
-/* we are clearing fs later just before "jmp resume_userspace",
+/* we are clearing gs later just before "jmp resume_userspace",
  * because it is not saved/restored.
  */
 
@@ -321,8 +322,8 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
  */
         info->regs32->eax = 0;
         tsk->thread.saved_esp0 = tsk->thread.esp0;
-        savesegment(fs, tsk->thread.saved_fs);
-        tsk->thread.saved_gs = info->regs32->xgs;
+        tsk->thread.saved_fs = info->regs32->xfs;
+        savesegment(gs, tsk->thread.saved_gs);
 
         tss = &per_cpu(init_tss, get_cpu());
         tsk->thread.esp0 = (unsigned long) &info->VM86_TSS_ESP0;
@@ -342,7 +343,7 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
         __asm__ __volatile__(
                 "movl %0,%%esp\n\t"
                 "movl %1,%%ebp\n\t"
-                "mov  %2, %%fs\n\t"
+                "mov  %2, %%gs\n\t"
                 "jmp resume_userspace"
                 : /* no outputs */
                 :"r" (&info->regs), "r" (task_thread_info(tsk)), "r" (0));
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: