x86: fix context switch on debug hypervisor after 1183:5e3c342a325e

Looking at anything but the result field of a multicall structure after
issuing the multicall is invalid - the debug hypervisor intentionally
clobbers all other fields.

On i386 also remove some left over debugging code.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/arch/i386/kernel/process-xen.c
+++ b/arch/i386/kernel/process-xen.c
@@ -548,7 +548,7 @@ struct task_struct fastcall * __switch_t
 {
 	struct thread_struct *prev = &prev_p->thread,
 				 *next = &next_p->thread;
-	int cpu = smp_processor_id();
+	int cpu = smp_processor_id(), cr0_ts;
 #ifndef CONFIG_X86_NO_TSS
 	struct tss_struct *tss = &per_cpu(init_tss, cpu);
 #endif
@@ -575,9 +575,6 @@ struct task_struct fastcall * __switch_t
 		mcl->args[0] = 1;
 		mcl++;
 	}
-#if 0 /* lazy fpu sanity check */
-	else BUG_ON(!(read_cr0() & 8));
-#endif
 
 	/*
 	 * Reload esp0.
@@ -639,11 +636,14 @@ struct task_struct fastcall * __switch_t
 	BUG_ON(pdo > _pdo + ARRAY_SIZE(_pdo));
 #endif
 	BUG_ON(mcl > _mcl + ARRAY_SIZE(_mcl));
-	if (_mcl->op == __HYPERVISOR_fpu_taskswitch)
+	if (_mcl->op == __HYPERVISOR_fpu_taskswitch) {
 		__get_cpu_var(xen_x86_cr0_upd) = X86_CR0_TS;
+		cr0_ts = 1;
+	} else
+		cr0_ts = 0;
 	if (unlikely(HYPERVISOR_multicall_check(_mcl, mcl - _mcl, NULL)))
 		BUG();
-	if (_mcl->op == __HYPERVISOR_fpu_taskswitch) {
+	if (cr0_ts) {
 		__get_cpu_var(xen_x86_cr0) |= X86_CR0_TS;
 		xen_clear_cr0_upd();
 	}
--- a/arch/x86_64/kernel/process-xen.c
+++ b/arch/x86_64/kernel/process-xen.c
@@ -486,7 +486,7 @@ __switch_to(struct task_struct *prev_p, 
 {
 	struct thread_struct *prev = &prev_p->thread,
 				 *next = &next_p->thread;
-	int cpu = smp_processor_id();  
+	int cpu = smp_processor_id(), cr0_ts;
 #ifndef CONFIG_X86_NO_TSS
 	struct tss_struct *tss = &per_cpu(init_tss, cpu);
 #endif
@@ -572,11 +572,14 @@ __switch_to(struct task_struct *prev_p, 
 	BUG_ON(pdo > _pdo + ARRAY_SIZE(_pdo));
 #endif
 	BUG_ON(mcl > _mcl + ARRAY_SIZE(_mcl));
-	if (_mcl->op == __HYPERVISOR_fpu_taskswitch)
+	if (_mcl->op == __HYPERVISOR_fpu_taskswitch) {
 		__get_cpu_var(xen_x86_cr0_upd) = X86_CR0_TS;
+		cr0_ts = 1;
+	} else
+		cr0_ts = 0;
 	if (unlikely(HYPERVISOR_multicall_check(_mcl, mcl - _mcl, NULL)))
 		BUG();
-	if (_mcl->op == __HYPERVISOR_fpu_taskswitch) {
+	if (cr0_ts) {
 		__get_cpu_var(xen_x86_cr0) |= X86_CR0_TS;
 		xen_clear_cr0_upd();
 	}