KVM: selftests: Run dirty ring test asynchronously

Previously the dirty ring test was working in synchronous way, because
only with a vmexit (with that it was the ring full event) we'll know
the hardware dirty bits will be flushed to the dirty ring.

With this patch we first introduce a vcpu kick mechanism using SIGUSR1,
which guarantees a vmexit and also therefore the flushing of hardware
dirty bits.  Once this is in place, we can keep the vcpu dirty work
asynchronous of the whole collection procedure now.  Still, we need
to be very careful that when reaching the ring buffer soft limit
(KVM_EXIT_DIRTY_RING_FULL) we must collect the dirty bits before
continuing the vcpu.

Further increase the dirty ring size to current maximum to make sure
we torture more on the no-ring-full case, which should be the major
scenario when the hypervisors like QEMU would like to use this feature.
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012239.6159-1-peterx@redhat.com>
[Use KVM_SET_SIGNAL_MASK+sigwait instead of a signal handler. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/dirty_log_test.c

@@ -62,7 +62,9 @@
 # define test_and_clear_bit_le	test_and_clear_bit
 #endif
 
-#define TEST_DIRTY_RING_COUNT		1024
+#define TEST_DIRTY_RING_COUNT		65536
+
+#define SIG_IPI SIGUSR1
 
 /*
  * Guest/Host shared variables. Ensure addr_gva2hva() and/or
@@ -138,6 +140,12 @@ static uint64_t host_track_next_count;
 /* Whether dirty ring reset is requested, or finished */
 static sem_t dirty_ring_vcpu_stop;
 static sem_t dirty_ring_vcpu_cont;
+/*
+ * This is updated by the vcpu thread to tell the host whether it's a
+ * ring-full event.  It should only be read until a sem_wait() of
+ * dirty_ring_vcpu_stop and before vcpu continues to run.
+ */
+static bool dirty_ring_vcpu_ring_full;
 /*
  * This is only used for verifying the dirty pages.  Dirty ring has a very
  * tricky case when the ring just got full, kvm will do userspace exit due to
@@ -176,6 +184,11 @@ static enum log_mode_t host_log_mode_option = LOG_MODE_ALL;
 static enum log_mode_t host_log_mode;
 static pthread_t vcpu_thread;
 
+static void vcpu_kick(void)
+{
+	pthread_kill(vcpu_thread, SIG_IPI);
+}
+
 /*
  * In our test we do signal tricks, let's use a better version of
  * sem_wait to avoid signal interrupts
@@ -286,6 +299,8 @@ static uint32_t dirty_ring_collect_one(struct kvm_dirty_gfn *dirty_gfns,
 
 static void dirty_ring_wait_vcpu(void)
 {
+	/* This makes sure that hardware PML cache flushed */
+	vcpu_kick();
 	sem_wait_until(&dirty_ring_vcpu_stop);
 }
 
@@ -301,9 +316,19 @@ static void dirty_ring_collect_dirty_pages(struct kvm_vm *vm, int slot,
 	/* We only have one vcpu */
 	static uint32_t fetch_index = 0;
 	uint32_t count = 0, cleared;
+	bool continued_vcpu = false;
 
 	dirty_ring_wait_vcpu();
 
+	if (!dirty_ring_vcpu_ring_full) {
+		/*
+		 * This is not a ring-full event, it's safe to allow
+		 * vcpu to continue
+		 */
+		dirty_ring_continue_vcpu();
+		continued_vcpu = true;
+	}
+
 	/* Only have one vcpu */
 	count = dirty_ring_collect_one(vcpu_map_dirty_ring(vm, VCPU_ID),
 				       slot, bitmap, num_pages, &fetch_index);
@@ -314,7 +339,11 @@ static void dirty_ring_collect_dirty_pages(struct kvm_vm *vm, int slot,
 	TEST_ASSERT(cleared == count, "Reset dirty pages (%u) mismatch "
 		    "with collected (%u)", cleared, count);
 
-	dirty_ring_continue_vcpu();
+	if (!continued_vcpu) {
+		TEST_ASSERT(dirty_ring_vcpu_ring_full,
+			    "Didn't continue vcpu even without ring full");
+		dirty_ring_continue_vcpu();
+	}
 
 	pr_info("Iteration %ld collected %u pages\n", iteration, count);
 }
@@ -327,10 +356,15 @@ static void dirty_ring_after_vcpu_run(struct kvm_vm *vm, int ret, int err)
 	if (get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC) {
 		/* We should allow this to continue */
 		;
-	} else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL) {
+	} else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL ||
+		   (ret == -1 && err == EINTR)) {
 		/* Update the flag first before pause */
+		WRITE_ONCE(dirty_ring_vcpu_ring_full,
+			   run->exit_reason == KVM_EXIT_DIRTY_RING_FULL);
 		sem_post(&dirty_ring_vcpu_stop);
-		pr_info("vcpu stops because dirty ring is full...\n");
+		pr_info("vcpu stops because %s...\n",
+			dirty_ring_vcpu_ring_full ?
+			"dirty ring is full" : "vcpu is kicked out");
 		sem_wait_until(&dirty_ring_vcpu_cont);
 		pr_info("vcpu continues now.\n");
 	} else {
@@ -458,9 +492,26 @@ static void *vcpu_worker(void *data)
 	struct kvm_vm *vm = data;
 	uint64_t *guest_array;
 	uint64_t pages_count = 0;
+	struct kvm_signal_mask *sigmask = alloca(offsetof(struct kvm_signal_mask, sigset)
+						 + sizeof(sigset_t));
+	sigset_t *sigset = (sigset_t *) &sigmask->sigset;
 
 	vcpu_fd = vcpu_get_fd(vm, VCPU_ID);
 
+	/*
+	 * SIG_IPI is unblocked atomically while in KVM_RUN.  It causes the
+	 * ioctl to return with -EINTR, but it is still pending and we need
+	 * to accept it with the sigwait.
+	 */
+	sigmask->len = 8;
+	pthread_sigmask(0, NULL, sigset);
+	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);
+
 	guest_array = addr_gva2hva(vm, (vm_vaddr_t)random_array);
 
 	while (!READ_ONCE(host_quit)) {
@@ -469,6 +520,11 @@ static void *vcpu_worker(void *data)
 		pages_count += TEST_PAGES_PER_LOOP;
 		/* Let the guest dirty the random pages */
 		ret = ioctl(vcpu_fd, KVM_RUN, NULL);
+		if (ret == -1 && errno == EINTR) {
+			int sig = -1;
+			sigwait(sigset, &sig);
+			assert(sig == SIG_IPI);
+		}
 		log_mode_after_vcpu_run(vm, ret, errno);
 	}