KVM: Don't spin on virt instruction faults during reboot

Since vmx blocks INIT signals, we disable virtualization extensions during
reboot.  This leads to virtualization instructions faulting; we trap these
faults and spin while the reboot continues.

Unfortunately spinning on a non-preemptible kernel may block a task that
reboot depends on; this causes the reboot to hang.

Fix by skipping over the instruction and hoping for the best.
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -786,14 +786,18 @@ enum {
  * reboot turns off virtualization while processes are running.
  * Trap the fault and ignore the instruction if that happens.
  */
-asmlinkage void kvm_handle_fault_on_reboot(void);
+asmlinkage void kvm_spurious_fault(void);
+extern bool kvm_rebooting;
 
 #define __kvm_handle_fault_on_reboot(insn) \
 	"666: " insn "\n\t" \
+	"668: \n\t"                           \
 	".pushsection .fixup, \"ax\" \n" \
 	"667: \n\t" \
+	"cmpb $0, kvm_rebooting \n\t"	      \
+	"jne 668b \n\t"      		      \
 	__ASM_SIZE(push) " $666b \n\t"	      \
-	"jmp kvm_handle_fault_on_reboot \n\t" \
+	"call kvm_spurious_fault \n\t"	      \
 	".popsection \n\t" \
 	".pushsection __ex_table, \"a\" \n\t" \
 	_ASM_PTR " 666b, 667b \n\t" \

virt/kvm/kvm_main.c

@@ -90,7 +90,8 @@ static void hardware_disable_all(void);
 
 static void kvm_io_bus_destroy(struct kvm_io_bus *bus);
 
-static bool kvm_rebooting;
+bool kvm_rebooting;
+EXPORT_SYMBOL_GPL(kvm_rebooting);
 
 static bool largepages_enabled = true;
 
@@ -2171,18 +2172,12 @@ static int kvm_cpu_hotplug(struct notifier_block *notifier, unsigned long val,
 }
 
 
-asmlinkage void kvm_handle_fault_on_reboot(void)
+asmlinkage void kvm_spurious_fault(void)
 {
-	if (kvm_rebooting) {
-		/* spin while reset goes on */
-		local_irq_enable();
-		while (true)
-			cpu_relax();
-	}
 	/* Fault while not rebooting.  We want the trace. */
 	BUG();
 }
-EXPORT_SYMBOL_GPL(kvm_handle_fault_on_reboot);
+EXPORT_SYMBOL_GPL(kvm_spurious_fault);
 
 static int kvm_reboot(struct notifier_block *notifier, unsigned long val,
 		      void *v)