linux-kernel - Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM pointer invalidated

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Date:   Tue, 8 Dec 2020 01:01:25 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, borntraeger@...ibm.com, cohuck@...hat.com,
        alex.williamson@...hat.com, kwankhede@...dia.com, david@...hat.com,
        Janosch Frank <frankja@...ux.ibm.com>
Subject: Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM
 pointer invalidated

On Mon, 7 Dec 2020 13:50:36 -0500
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> On 12/4/20 2:05 PM, Halil Pasic wrote:
> > On Fri, 4 Dec 2020 09:43:59 -0500
> > Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> >  
> >>>> +{
> >>>> +	if (matrix_mdev->kvm) {
> >>>> +		(matrix_mdev->kvm);
> >>>> +		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;  
> >>> Is a plain assignment to arch.crypto.pqap_hook apropriate, or do we need
> >>> to take more care?
> >>>
> >>> For instance kvm_arch_crypto_set_masks() takes kvm->lock before poking
> >>> kvm->arch.crypto.crycb.  
> >> I do not think so. The CRYCB is used by KVM to provide crypto resources
> >> to the guest so it makes sense to protect it from changes to it while
> >> passing
> >> the AP devices through to the guest. The hook is used only when an AQIC
> >> executed on the guest is intercepted by KVM. If the notifier
> >> is being invoked to notify vfio_ap that KVM has been set to NULL, this means
> >> the guest is gone in which case there will be no AP instructions to
> >> intercept.  
> > If the update to pqap_hook isn't observed as atomic we still have a
> > problem. With torn writes or reads we would try to use a corrupt function
> > pointer. While the compiler probably ain't likely to generate silly code
> > for the above assignment (multiple write instructions less then
> > quadword wide), I know of nothing that would prohibit the compiler to do
> > so.  
> 
> I'm sorry, but I still don't understand why you tkvm_vfio_group_set_kvmhink this is a problem
> given what I stated above.

I assume you are specifically referring to 'the guest is gone in which
case there will be no AP instructions to intercept'.  I assume by 'guest
is gone' you mean that the VM is being destroyed, and the vcpus are out
of SIE. You are probably right for the invocation of
kvm_vfio_group_set_kvm() in kvm_vfio_destroy(), but is that true for
the invocation in the KVM_DEV_VFIO_GROUP_DEL case in
kvm_vfio_set_group()? I.e. can't we get the notifier called when the
qemu device is hot unplugged (modulo remove which unregisters the
notifier and usually precludes the notifier being with NULL called at
all)?

Regards,
Halil