KVM: selftests: Set per-iteration variables at the start of each iteration

Set the per-iteration variables at the start of each iteration instead of
setting them before the loop, and at the end of each iteration.  To ensure
the vCPU doesn't race ahead before the first iteration, simply have the
vCPU worker want for sem_vcpu_cont, which conveniently avoids the need to
special case posting sem_vcpu_cont from the loop.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

tools/testing/selftests/kvm/dirty_log_test.c

@@ -481,6 +481,8 @@ static void *vcpu_worker(void *data)
 {
 	struct kvm_vcpu *vcpu = data;
 
+	sem_wait(&sem_vcpu_cont);
+
 	while (!READ_ONCE(host_quit)) {
 		/* Let the guest dirty the random pages */
 		vcpu_run(vcpu);
@@ -675,15 +677,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	sync_global_to_guest(vm, guest_test_virt_mem);
 	sync_global_to_guest(vm, guest_num_pages);
 
-	/* Start the iterations */
-	iteration = 1;
-	sync_global_to_guest(vm, iteration);
-	WRITE_ONCE(host_quit, false);
 	host_dirty_count = 0;
 	host_clear_count = 0;
-	WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
-	WRITE_ONCE(nr_writes, 0);
-	sync_global_to_guest(vm, nr_writes);
+	WRITE_ONCE(host_quit, false);
 
 	/*
 	 * Ensure the previous iteration didn't leave a dangling semaphore, i.e.
@@ -695,12 +691,22 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	sem_getvalue(&sem_vcpu_cont, &sem_val);
 	TEST_ASSERT_EQ(sem_val, 0);
 
+	TEST_ASSERT_EQ(vcpu_stop, false);
+
 	pthread_create(&vcpu_thread, NULL, vcpu_worker, vcpu);
 
-	while (iteration < p->iterations) {
+	for (iteration = 1; iteration < p->iterations; iteration++) {
 		unsigned long i;
 
+		sync_global_to_guest(vm, iteration);
+
+		WRITE_ONCE(nr_writes, 0);
+		sync_global_to_guest(vm, nr_writes);
+
 		dirty_ring_prev_iteration_last_page = dirty_ring_last_page;
+		WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
+
+		sem_post(&sem_vcpu_cont);
 
 		/*
 		 * Let the vCPU run beyond the configured interval until it has
@@ -785,26 +791,11 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 					     bmap[1], host_num_pages,
 					     &ring_buf_idx);
 		vm_dirty_log_verify(mode, bmap);
-
-		/*
-		 * Set host_quit before sem_vcpu_cont in the final iteration to
-		 * ensure that the vCPU worker doesn't resume the guest.  As
-		 * above, the dirty ring test may stop and wait even when not
-		 * explicitly request to do so, i.e. would hang waiting for a
-		 * "continue" if it's allowed to resume the guest.
-		 */
-		if (++iteration == p->iterations)
-			WRITE_ONCE(host_quit, true);
-		sync_global_to_guest(vm, iteration);
-
-		WRITE_ONCE(nr_writes, 0);
-		sync_global_to_guest(vm, nr_writes);
-
-		WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
-
-		sem_post(&sem_vcpu_cont);
 	}
 
+	WRITE_ONCE(host_quit, true);
+	sem_post(&sem_vcpu_cont);
+
 	pthread_join(vcpu_thread, NULL);
 
 	pr_info("Total bits checked: dirty (%lu), clear (%lu)\n",