Re: [PATCH 02/20] KVM: selftests: Sync dirty_log_test iteration to guest *before* resuming

Maxim Levitsky
2024-12-19T15:11:07Z

On Wed, 2024-12-18 at 13:36 -0800, Sean Christopherson wrote:
> On Tue, Dec 17, 2024, Maxim Levitsky wrote:
> > On Fri, 2024-12-13 at 17:07 -0800, Sean Christopherson wrote:
> > > Sync the new iteration to the guest prior to restarting the vCPU, otherwise
> > > it's possible for the vCPU to dirty memory for the next iteration using the
> > > current iteration's value.
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > > tools/testing/selftests/kvm/dirty_log_test.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> > > index cdae103314fc..41c158cf5444 100644
> > > --- a/tools/testing/selftests/kvm/dirty_log_test.c
> > > +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> > > @@ -859,9 +859,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> > > */
> > > if (++iteration == p->iterations)
> > > WRITE_ONCE(host_quit, true);
> > > -
> > > - sem_post(&sem_vcpu_cont);
> > > sync_global_to_guest(vm, iteration);
> > > +
> > > + sem_post(&sem_vcpu_cont);
> > > }
> > >
> > > pthread_join(vcpu_thread, NULL);
> >
> > AFAIK, this patch doesn't 100% gurantee that this won't happen:
> >
> > The READ_ONCE that guest uses only guarntees no wierd compiler optimizations
> > are used. The guest can still read the iteration value to a register, get
> > #vmexit, after which the iteration will be increased and then write the old
> > value.
>
> Hmm, right, it's not 100% guaranteed because of the register caching angle. But
> it does guarantee that at most only write can retire with the previous iteration,
> and patch 1 from you addresses that issue, so I think this is solid?
>
> Assuming we end up going with the "collect everything for the current iteration",
> I'll expand the changelog to call out the dependency along with exactly what
> protection this does and does not provide
>
> > Is this worth to reorder this to decrease the chances of this happening? I am
> > not sure, as this will just make this problem rarer and thus harder to debug.
> > Currently the test just assumes that this can happen and deals with this.
>
> The test deals with it by effectively disabling verification. IMO, that's just
> hacking around a bug.
>

OK, let it be, but the changelog needs to be updated to state that the race is still
possible.

Best regards,
Maxim Levitsky