KVM: selftests: Honor "stop" request in dirty ring test

Now that the vCPU doesn't dirty every page on the first iteration for
architectures that support the dirty ring, honor vcpu_stop in the dirty
ring's vCPU worker, i.e. stop when the main thread says "stop".  This will
allow plumbing vcpu_stop into the guest so that the vCPU doesn't need to
periodically exit to userspace just to see if it should stop.

Add a comment explaining that marking all pages as dirty is problematic
for the dirty ring, as it results in the guest getting stuck on "ring
full".  This could be addressed by adding a GUEST_SYNC() in that initial
loop, but it's not clear how that would interact with s390's behavior.

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

@@ -387,8 +387,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vcpu *vcpu)
 
 	/* A ucall-sync or ring-full event is allowed */
 	if (get_ucall(vcpu, NULL) == UCALL_SYNC) {
-		/* We should allow this to continue */
-		;
+		vcpu_handle_sync_stop();
 	} else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL) {
 		/* Update the flag first before pause */
 		WRITE_ONCE(dirty_ring_vcpu_ring_full, true);
@@ -697,6 +696,15 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 #ifdef __s390x__
 	/* Align to 1M (segment size) */
 	guest_test_phys_mem = align_down(guest_test_phys_mem, 1 << 20);
+
+	/*
+	 * The workaround in guest_code() to write all pages prior to the first
+	 * iteration isn't compatible with the dirty ring, as the dirty ring
+	 * support relies on the vCPU to actually stop when vcpu_stop is set so
+	 * that the vCPU doesn't hang waiting for the dirty ring to be emptied.
+	 */
+	TEST_ASSERT(host_log_mode != LOG_MODE_DIRTY_RING,
+		    "Test needs to be updated to support s390 dirty ring");
 #endif
 
 	pr_info("guest physical test memory offset: 0x%lx\n", guest_test_phys_mem);