Re: [PATCH v17 08/15] s390/vfio-ap: keep track of active guests

[Tony Krowiak]

On 12/29/21 21:04, Halil Pasic wrote:
> On Thu, 21 Oct 2021 11:23:25 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > The reason a lockdep splat can occur has to do with the fact that the
> > kvm->lock has to be taken before the vcpu->lock; so, for example, when a
> > secure execution guest is started, you may end up with the following
> > scenario:
> >
> >          Interception of PQAP(AQIC) instruction executed on the guest:
> >          ------------------------------------------------------------
> >          handle_pqap:                    matrix_dev->lock
> >          kvm_vcpu_ioctl:                 vcpu_mutex
> >
> >          Start of secure execution guest:
> >          -------------------------------
> >          kvm_s390_cpus_to_pv:            vcpu->mutex
> >          kvm_arch_vm_ioctl:              kvm->lock
> >
> >          Queue is unbound from vfio_ap device driver:
> >          -------------------------------------------
> >                                          kvm->lock
> >          vfio_ap_mdev_remove_queue:      matrix_dev->lock
> The way you describe your scenario is a little ambiguous. It
> seems you choose a stack-trace like description, in a sense that for
> example for PQAP: first vcpu->mutex is taken and then matrix_dev->lock
> but you write the latter first and the former second. I think it is more
> usual to describe such stuff a a sequence of event in a sense that
> if A precedes B in the text (from the top towards the bottom), then
> execution of a A precedes the execution of B in time.

I wrote it the way it is displayed in the lockdep splat trace.
I'd be happy to re-arrange it if you'd prefer.

> Also you are inconsistent with vcpu_mutex vs vcpu->mutex.
>
> I can't say I understand the need for this yet. I have been starring
> at the end result for a while. Let me see if I can come up with an
> alternate proposal for some things.

Go for it, and may the force be with you.

> Regards,
> Halil