[BUG] 2.5.65: Caching MSR_IA32_SYSENTER_CS kills dosemu

[Wayne Whitney]

[This is a repost with a more descriptive subject.]

I run dosemu 1.0.2.1 on the 2.5.x kernels.  Upgrading from 2.5.64 to
2.5.65 (or 2.5.66) causes dosemu to no longer work:  it locks up the
machine shortly after I run it.  Alt-Sysrq still works OK.  I traced the
problem to the "Cache MSR_IA32_SYSENTER_CS value in the per-CPU TSS"
changeset introduced in 2.5.65-bk5, which I have included as a patch
below.  Backing this patch out from 2.5.65-bk5 makes dosemu work again.  
Any idea what is going on?

Cheers, Wayne

diff -ru linux-2.5.64-bk4/arch/i386/kernel/sysenter.c linux-2.5.64-bk5/arch/i386/kernel/sysenter.c
--- linux-2.5.64-bk4/arch/i386/kernel/sysenter.c	2003-03-26 21:06:54.000000000 -0800
+++ linux-2.5.64-bk5/arch/i386/kernel/sysenter.c	2003-03-26 13:55:08.000000000 -0800
@@ -40,6 +40,7 @@
 	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);
diff -ru linux-2.5.64-bk4/arch/i386/kernel/vm86.c linux-2.5.64-bk5/arch/i386/kernel/vm86.c
--- linux-2.5.64-bk4/arch/i386/kernel/vm86.c	2003-03-26 21:06:54.000000000 -0800
+++ linux-2.5.64-bk5/arch/i386/kernel/vm86.c	2003-03-26 13:55:08.000000000 -0800
@@ -291,7 +291,7 @@
 
 	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)
diff -ru linux-2.5.64-bk4/include/asm-i386/processor.h linux-2.5.64-bk5/include/asm-i386/processor.h
--- linux-2.5.64-bk4/include/asm-i386/processor.h	2003-03-26 21:06:55.000000000 -0800
+++ linux-2.5.64-bk5/include/asm-i386/processor.h	2003-03-26 13:55:09.000000000 -0800
@@ -347,7 +347,7 @@
 	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 @@
 {
 	tss->esp0 = esp0;
 	if (cpu_has_sep) {
-		wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+		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 {		\