KVM: selftests: Always run vCPU thread with blocked SIG_IPI

The main thread could start to send SIG_IPI at any time, even before signal
blocked on vcpu thread.  Therefore, start the vcpu thread with the signal
blocked.

Without this patch, on very busy cores the dirty_log_test could fail directly
on receiving a SIGUSR1 without a handler (when vcpu runs far slower than main).
Reported-by: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/dirty_log_test.c

@@ -527,9 +527,8 @@ static void *vcpu_worker(void *data)
 	 */
 	sigmask->len = 8;
 	pthread_sigmask(0, NULL, sigset);
+	sigdelset(sigset, SIG_IPI);
 	vcpu_ioctl(vm, VCPU_ID, KVM_SET_SIGNAL_MASK, sigmask);
-	sigaddset(sigset, SIG_IPI);
-	pthread_sigmask(SIG_BLOCK, sigset, NULL);
 
 	sigemptyset(sigset);
 	sigaddset(sigset, SIG_IPI);
@@ -858,6 +857,7 @@ int main(int argc, char *argv[])
 		.interval = TEST_HOST_LOOP_INTERVAL,
 	};
 	int opt, i;
+	sigset_t sigset;
 
 	sem_init(&sem_vcpu_stop, 0, 0);
 	sem_init(&sem_vcpu_cont, 0, 0);
@@ -916,6 +916,11 @@ int main(int argc, char *argv[])
 
 	srandom(time(0));
 
+	/* Ensure that vCPU threads start with SIG_IPI blocked.  */
+	sigemptyset(&sigset);
+	sigaddset(&sigset, SIG_IPI);
+	pthread_sigmask(SIG_BLOCK, &sigset, NULL);
+
 	if (host_log_mode_option == LOG_MODE_ALL) {
 		/* Run each log mode */
 		for (i = 0; i < LOG_MODE_NUM; i++) {