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authorJeremy Fitzhardinge <jeremy@goop.org>2007-02-13 13:26:20 +0100
committerAndi Kleen <andi@basil.nowhere.org>2007-02-13 13:26:20 +0100
commit464d1a78fbf8cf6c7fd970e7b3e2db50a320ce28 (patch)
tree536d8a92976e675b484b35dec88d40c97fab8ac8 /arch/i386
parent54413927f022292aeccadd268fbf1c0b42129945 (diff)
[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>
Diffstat (limited to 'arch/i386')
-rw-r--r--arch/i386/kernel/asm-offsets.c2
-rw-r--r--arch/i386/kernel/cpu/common.c14
-rw-r--r--arch/i386/kernel/entry.S32
-rw-r--r--arch/i386/kernel/head.S6
-rw-r--r--arch/i386/kernel/kprobes.c4
-rw-r--r--arch/i386/kernel/process.c24
-rw-r--r--arch/i386/kernel/ptrace.c16
-rw-r--r--arch/i386/kernel/signal.c10
-rw-r--r--arch/i386/kernel/traps.c7
-rw-r--r--arch/i386/kernel/vm86.c33
-rw-r--r--arch/i386/math-emu/get_address.c14
11 files changed, 79 insertions, 83 deletions
diff --git a/arch/i386/kernel/asm-offsets.c b/arch/i386/kernel/asm-offsets.c
index 1b2f3cd3327..c37535163bf 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 8a8bbdaaf38..dcbbd0a8bfc 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 5e47683fc63..8c6a22a42d2 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 cb9abdfced9..15336c8b596 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 af1d5334499..b85cfa3ce1d 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 c641056233a..23ae198dbbc 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 af8aabe8580..4a8f8a25972 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 65d7620eaa0..8f4afcc7d2a 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 0efad8aeb41..4ec21037a36 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 be2f96e67f7..d1b8f2b7aea 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 9819b705efa..2e2c51a8bd3 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: