linux-kernel - Re: [PATCH v13 13/14] KVM: selftests: Let dirty_log

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Date:   Fri, 6 Nov 2020 12:27:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v13 13/14] KVM: selftests: Let dirty_log_test async for
 dirty ring test

On 01/10/20 03:22, Peter Xu wrote:
> +
> +static void vcpu_sig_handler(int sig)
> +{
> +	TEST_ASSERT(sig == SIG_IPI, "unknown signal: %d", sig);
> +}
> +

Unless you also use run->immediate_exit in vcpu_kick, this is racy.  The 
alternative is to _not_ set up a signal handler and instead block the 
signal.  KVM_SET_SIGNAL_MASK unblocks the signal inside the VM and on 
-EINTR sigwait accepts the signal (removes it from the set of pending 
signal).

This is a bit more complicated, but I think it's a good idea to do it 
this way for documentation purposes.  Here is the patch:

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c 
b/tools/testing/selftests/kvm/dirty_log_test.c
index 4b404dfdc2f9..9a5b876b74af 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -172,11 +172,6 @@ static pthread_t vcpu_thread;
  /* Only way to pass this to the signal handler */
  static struct kvm_vm *current_vm;

-static void vcpu_sig_handler(int sig)
-{
-	TEST_ASSERT(sig == SIG_IPI, "unknown signal: %d", sig);
-}
-
  static void vcpu_kick(void)
  {
  	pthread_kill(vcpu_thread, SIG_IPI);
@@ -484,13 +479,26 @@ static void *vcpu_worker(void *data)
  	struct kvm_vm *vm = data;
  	uint64_t *guest_array;
  	uint64_t pages_count = 0;
-	struct sigaction sigact;
+	struct kvm_signal_mask *sigmask = alloca(offsetof(struct 
kvm_signal_mask, sigset)
+						 + sizeof(sigset_t));
+	sigset_t *sigset = (sigset_t *) &sigmask->sigset;

  	current_vm = vm;
  	vcpu_fd = vcpu_get_fd(vm, VCPU_ID);
-	memset(&sigact, 0, sizeof(sigact));
-	sigact.sa_handler = vcpu_sig_handler;
-	sigaction(SIG_IPI, &sigact, NULL);
+
+	/*
+	 * 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);

@@ -500,6 +508,11 @@ 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 == -EINTR) {
+			int sig = -1;
+			sigwait(sigset, &sig);
+			assert(sig == SIG_IPI);
+		}
  		log_mode_after_vcpu_run(vm, ret, errno);
  	}