KVM: Fix hang on uniprocessor

This is not in mainline, as it was fixed differently in that tree.

first_cpu(cpus) returns the only CPU when NR_CPUS is 1 regardless of
the cpus mask. Therefore we avoid a kernel hang in
KVM_SET_MEMORY_REGION ioctl on uniprocessor by not entering the loop at
all.
Signed-off-by: Marko Kohtala <marko.kohtala@gmail.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/kvm/kvm_main.c

@@ -273,6 +273,11 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
 			}
 	}
 
+	/* Uniprocessor kernel does not respect cpus in first_cpu. So
+	 * do not go there if we have nothing to do. */
+	if (cpus_empty(cpus))
+		return;
+
 	/*
 	 * We really want smp_call_function_mask() here.  But that's not
 	 * available, so ipi all cpus in parallel and wait for them