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" \