Fix the remaining known problems (NMI and debugging) with

fast system calls. They should now be fully comparable to
traditional system calls.

    Fix SEP stack allocation to look like a real thread stack.
    
    This fixes the "Trap on first sysenter instruction" problem,
    which can happen either as a result of an NMI or a breakpoint
    trap (TF bit set).
    
    Also make the user trampoline save and restore EFLAGS, so that
    the TF bit gets restored after a fast system call (it will have
    been cleared to avoid trap faults in the kernel).

arch/i386/kernel/entry.S

@@ -233,7 +233,7 @@ need_resched:
 #endif
 
 /* Points to after the "sysenter" instruction in the vsyscall page */
-#define SYSENTER_RETURN 0xffffe009
+#define SYSENTER_RETURN 0xffffe00a
 
 	# sysenter call handler stub
 	ALIGN

arch/i386/kernel/sysenter.c

@@ -9,6 +9,7 @@
 #include <linux/init.h>
 #include <linux/smp.h>
 #include <linux/thread_info.h>
+#include <linux/sched.h>
 #include <linux/gfp.h>
 #include <linux/string.h>
 
@@ -18,24 +19,55 @@
 
 extern asmlinkage void sysenter_entry(void);
 
+/*
+ * Create a per-cpu fake "SEP thread" stack, so that we can
+ * enter the kernel without having to worry about things like
+ * "current" etc not working (debug traps and NMI's can happen
+ * before we can switch over to the "real" thread).
+ *
+ * Return the resulting fake stack pointer.
+ */
+struct fake_sep_struct {
+	struct thread_info thread;
+	struct task_struct task;
+	unsigned char trampoline[32] __attribute__((aligned(1024)));
+	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)
 {
-	unsigned long page = __get_free_page(GFP_ATOMIC);
 	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 - (page+11);
-
+	rel32 = (unsigned long) sysenter_entry - (unsigned long) (sep->trampoline+11);
 	
-	*(short *) (page+0) = 0x258b;		/* movl xxxxx,%esp */
-	*(long **) (page+2) = esp0_ptr;
-	*(char *)  (page+6) = 0xe9;		/* jmp rl32 */
-	*(long *)  (page+7) = rel32;
+	*(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;
 
 	wrmsr(0x174, __KERNEL_CS, 0);				/* SYSENTER_CS_MSR */
-	wrmsr(0x175, page+PAGE_SIZE, 0);	/* SYSENTER_ESP_MSR */
-	wrmsr(0x176, page, 0);			/* SYSENTER_EIP_MSR */
+	wrmsr(0x175, PAGE_SIZE*2 + (unsigned long) sep, 0);	/* SYSENTER_ESP_MSR */
+	wrmsr(0x176, (unsigned long) &sep->trampoline, 0);	/* SYSENTER_EIP_MSR */
 
 	printk("Enabling SEP on CPU %d\n", cpu);
 	put_cpu();	
@@ -48,6 +80,7 @@ static int __init sysenter_setup(void)
 		0xc3			/* ret */
 	};
 	static const char sysent[] = {
+		0x9c,			/* pushf */
 		0x51,			/* push %ecx */
 		0x52,			/* push %edx */
 		0x55,			/* push %ebp */
@@ -59,6 +92,7 @@ static int __init sysenter_setup(void)
 		0x5d,			/* pop %ebp */
 		0x5a,			/* pop %edx */
 		0x59,			/* pop %ecx */
+		0x9d,			/* popf - restore TF */
 		0xc3			/* ret */
 	};
 	unsigned long page = get_zeroed_page(GFP_ATOMIC);