Cache the MSR_IA32_SYSENTER_CS value in the per-CPU TSS (using

the otherwise unused cpl1 entry for SS), so that we can avoid
re-loading it on task switches if it doesn't change.

arch/i386/kernel/sysenter.c

@@ -40,6 +40,7 @@ void enable_sep_cpu(void *info)
 	int cpu = get_cpu();
 	struct tss_struct *tss = init_tss + cpu;
 
+	tss->ss1 = __KERNEL_CS;
 	wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
 	wrmsr(MSR_IA32_SYSENTER_ESP, tss->esp0, 0);
 	wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) sysenter_entry, 0);

arch/i386/kernel/vm86.c

@@ -291,7 +291,7 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 
 	tss = init_tss + smp_processor_id();
 	tss->esp0 = tsk->thread.esp0 = (unsigned long) &info->VM86_TSS_ESP0;
-	disable_sysenter();
+	disable_sysenter(tss);
 
 	tsk->thread.screen_bitmap = info->screen_bitmap;
 	if (info->flags & VM86_SCREEN_BITMAP)

include/asm-i386/processor.h

@@ -347,7 +347,7 @@ struct tss_struct {
 	unsigned long	esp0;
 	unsigned short	ss0,__ss0h;
 	unsigned long	esp1;
-	unsigned short	ss1,__ss1h;
+	unsigned short	ss1,__ss1h;	/* ss1 is used to cache MSR_IA32_SYSENTER_CS */
 	unsigned long	esp2;
 	unsigned short	ss2,__ss2h;
 	unsigned long	__cr3;
@@ -413,15 +413,20 @@ static inline void load_esp0(struct tss_struct *tss, unsigned long esp0)
 {
 	tss->esp0 = esp0;
 	if (cpu_has_sep) {
+		if (tss->ss1 != __KERNEL_CS) {
+			tss->ss1 = __KERNEL_CS;
 			wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+		}
 		wrmsr(MSR_IA32_SYSENTER_ESP, esp0, 0);
 	}
 }
 
-static inline void disable_sysenter(void)
+static inline void disable_sysenter(struct tss_struct *tss)
 {
-	if (cpu_has_sep)  
+	if (cpu_has_sep)  {
+		tss->ss1 = 0;
 		wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
+	}
 }
 
 #define start_thread(regs, new_eip, new_esp) do {		\