linux-kernel - Re: [PATCH v3 1/2] KVM: selftests: Sync data verify of dirty logging with guest sync

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Message-ID: <20210420131041.GZ4440@xz-x1>
Date:   Tue, 20 Apr 2021 09:10:41 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Andrew Jones <drjones@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v3 1/2] KVM: selftests: Sync data verify of dirty logging
 with guest sync

On Tue, Apr 20, 2021 at 10:07:16AM +0200, Paolo Bonzini wrote:
> On 18/04/21 14:43, Peter Xu wrote:
> > ----8<-----
> > diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> > index 25230e799bc4..d3050d1c2cd0 100644
> > --- a/tools/testing/selftests/kvm/dirty_log_test.c
> > +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> > @@ -377,7 +377,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vm *vm, int ret, int err)
> >          /* A ucall-sync or ring-full event is allowed */
> >          if (get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC) {
> >                  /* We should allow this to continue */
> > -               ;
> > +               vcpu_handle_sync_stop();
> >          } else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL ||
> >                     (ret == -1 && err == EINTR)) {
> >                  /* Update the flag first before pause */
> > ----8<-----
> > 
> > That's my intention when I introduce vcpu_handle_sync_stop(), but forgot to
> > add...
> 
> And possibly even this (untested though):
> 
> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> index ffa4e2791926..918954f01cef 100644
> --- a/tools/testing/selftests/kvm/dirty_log_test.c
> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> @@ -383,6 +383,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vm *vm, int ret, int err)
>  		/* Update the flag first before pause */
>  		WRITE_ONCE(dirty_ring_vcpu_ring_full,
>  			   run->exit_reason == KVM_EXIT_DIRTY_RING_FULL);
> +		atomic_set(&vcpu_sync_stop_requested, false);
>  		sem_post(&sem_vcpu_stop);
>  		pr_info("vcpu stops because %s...\n",
>  			dirty_ring_vcpu_ring_full ?
> @@ -804,8 +805,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
>  		 * the flush of the last page, and since we handle the last
>  		 * page specially verification will succeed anyway.
>  		 */
> -		assert(host_log_mode == LOG_MODE_DIRTY_RING ||
> -		       atomic_read(&vcpu_sync_stop_requested) == false);
> +		assert(atomic_read(&vcpu_sync_stop_requested) == false);
>  		vm_dirty_log_verify(mode, bmap);
>  		sem_post(&sem_vcpu_cont);
> 
> You can submit all these as a separate patch.

But it could race, then?

        main thread                 vcpu thread
        -----------                 -----------
                                  ring full
                                    vcpu_sync_stop_requested=0
                                    sem_post(&sem_vcpu_stop)
     vcpu_sync_stop_requested=1
     sem_wait(&sem_vcpu_stop)
     assert(vcpu_sync_stop_requested==0)   <----

Thanks,

-- 
Peter Xu