Sysenter cleanups (originals by Brian Gerst, updated and expanded by me):

 - set up kernel stack pointer for sysenter at each context switch.
 - disable sysenter while in vm86 mode.
 - clean up mtrr number defines and SEP feature testing

arch/i386/kernel/cpu/common.c

@@ -516,7 +516,7 @@ void __init cpu_init (void)
 		BUG();
 	enter_lazy_tlb(&init_mm, current, cpu);
 
-	t->esp0 = thread->esp0;
+	load_esp0(t, thread->esp0);
 	set_tss_desc(cpu,t);
 	cpu_gdt_table[cpu][GDT_ENTRY_TSS].b &= 0xfffffdff;
 	load_TR_desc();

arch/i386/kernel/process.c

@@ -443,7 +443,7 @@ void __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	/*
 	 * Reload esp0, LDT and the page table pointer:
 	 */
-	tss->esp0 = next->esp0;
+	load_esp0(tss, next->esp0);
 
 	/*
 	 * Load the per-thread Thread-Local Storage descriptor.

arch/i386/kernel/sysenter.c

@@ -34,40 +34,14 @@ struct fake_sep_struct {
 	unsigned char stack[0];
 } __attribute__((aligned(8192)));
 	
-static struct fake_sep_struct *alloc_sep_thread(int cpu)
-{
-	struct fake_sep_struct *entry;
-
-	entry = (struct fake_sep_struct *) __get_free_pages(GFP_ATOMIC, 1);
-	if (!entry)
-		return NULL;
-
-	memset(entry, 0, PAGE_SIZE<<1);
-	entry->thread.task = &entry->task;
-	entry->task.thread_info = &entry->thread;
-	entry->thread.preempt_count = 1;
-	entry->thread.cpu = cpu;	
-
-	return entry;
-}
-
 static void __init enable_sep_cpu(void *info)
 {
 	int cpu = get_cpu();
-	struct fake_sep_struct *sep = alloc_sep_thread(cpu);
-	unsigned long *esp0_ptr = &(init_tss + cpu)->esp0;
-	unsigned long rel32;
-
-	rel32 = (unsigned long) sysenter_entry - (unsigned long) (sep->trampoline+11);
-	
-	*(short *) (sep->trampoline+0) = 0x258b;		/* movl xxxxx,%esp */
-	*(long **) (sep->trampoline+2) = esp0_ptr;
-	*(char *)  (sep->trampoline+6) = 0xe9;			/* jmp rl32 */
-	*(long *)  (sep->trampoline+7) = rel32;
+	struct tss_struct *tss = init_tss + cpu;
 
-	wrmsr(0x174, __KERNEL_CS, 0);				/* SYSENTER_CS_MSR */
-	wrmsr(0x175, PAGE_SIZE*2 + (unsigned long) sep, 0);	/* SYSENTER_ESP_MSR */
-	wrmsr(0x176, (unsigned long) &sep->trampoline, 0);	/* SYSENTER_EIP_MSR */
+	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);
 
 	printk("Enabling SEP on CPU %d\n", cpu);
 	put_cpu();	

arch/i386/kernel/vm86.c

@@ -113,7 +113,8 @@ struct pt_regs * save_v86_state(struct kernel_vm86_regs * regs)
 		do_exit(SIGSEGV);
 	}
 	tss = init_tss + smp_processor_id();
-	tss->esp0 = current->thread.esp0 = current->thread.saved_esp0;
+	current->thread.esp0 = current->thread.saved_esp0;
+	load_esp0(tss, current->thread.esp0);
 	current->thread.saved_esp0 = 0;
 	ret = KVM86->regs32;
 	return ret;
@@ -284,6 +285,7 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	tsk->thread.saved_esp0 = tsk->thread.esp0;
 	tss = init_tss + smp_processor_id();
 	tss->esp0 = tsk->thread.esp0 = (unsigned long) &info->VM86_TSS_ESP0;
+	disable_sysenter();
 
 	tsk->thread.screen_bitmap = info->screen_bitmap;
 	if (info->flags & VM86_SCREEN_BITMAP)

include/asm-i386/cpufeature.h

@@ -7,6 +7,8 @@
 #ifndef __ASM_I386_CPUFEATURE_H
 #define __ASM_I386_CPUFEATURE_H
 
+#include <linux/bitops.h>
+
 #define NCAPINTS	4	/* Currently we have 4 32-bit words worth of info */
 
 /* Intel-defined CPU features, CPUID level 0x00000001, word 0 */
@@ -74,6 +76,7 @@
 #define cpu_has_pae		boot_cpu_has(X86_FEATURE_PAE)
 #define cpu_has_pge		boot_cpu_has(X86_FEATURE_PGE)
 #define cpu_has_apic		boot_cpu_has(X86_FEATURE_APIC)
+#define cpu_has_sep		boot_cpu_has(X86_FEATURE_SEP)
 #define cpu_has_mtrr		boot_cpu_has(X86_FEATURE_MTRR)
 #define cpu_has_mmx		boot_cpu_has(X86_FEATURE_MMX)
 #define cpu_has_fxsr		boot_cpu_has(X86_FEATURE_FXSR)

include/asm-i386/msr.h

@@ -53,6 +53,10 @@
 
 #define MSR_IA32_BBL_CR_CTL		0x119
 
+#define MSR_IA32_SYSENTER_CS		0x174
+#define MSR_IA32_SYSENTER_ESP		0x175
+#define MSR_IA32_SYSENTER_EIP		0x176
+
 #define MSR_IA32_MCG_CAP		0x179
 #define MSR_IA32_MCG_STATUS		0x17a
 #define MSR_IA32_MCG_CTL		0x17b

include/asm-i386/processor.h

@@ -14,6 +14,7 @@
 #include <asm/types.h>
 #include <asm/sigcontext.h>
 #include <asm/cpufeature.h>
+#include <asm/msr.h>
 #include <linux/cache.h>
 #include <linux/config.h>
 #include <linux/threads.h>
@@ -416,6 +417,21 @@ struct thread_struct {
 	{~0, } /* ioperm */					\
 }
 
+static inline void load_esp0(struct tss_struct *tss, unsigned long esp0)
+{
+	tss->esp0 = esp0;
+	if (cpu_has_sep) {
+		wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+		wrmsr(MSR_IA32_SYSENTER_ESP, esp0, 0);
+	}
+}
+
+static inline void disable_sysenter(void)
+{
+	if (cpu_has_sep)  
+		wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
+}
+
 #define start_thread(regs, new_eip, new_esp) do {		\
 	__asm__("movl %0,%%fs ; movl %0,%%gs": :"r" (0));	\
 	set_fs(USER_DS);					\