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

[Paolo Bonzini]

On 20/04/21 15:10, Peter Xu wrote:
> 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)   <----

Yes, it could indeed.

Thanks,

Paolo