Re: [PATCH v5 4/7] s390: ap: setup relation betwen KVM and mediated device

[Pierre Morel]

On 19/03/2019 16:27, Halil Pasic wrote:
> On Tue, 19 Mar 2019 15:47:05 +0100
> Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
>
> > > > > > >  ÂÂÂÂÂÂ if (matrix_mdev->kvm)
> > > > > > >  ÂÂÂÂÂÂÂÂÂÂ kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> > > > > >
> > > > > > This still conditional?
> > > > >
> > > > > Yes, nothing to clear if there is no KVM.
> > > > >   
> > > >
> > > > Since we have ensured the open only works if there is a KVM at that
> > > > point in time, and we have taken a reference to KVM, I would expect
> > > > KVM can not go away before we give up our reference.
> > >
> > > Right.
> >
> > Right but based on the assumption we do a kvm_get_kvm() during open.
> >
> > But now we will do it inside the notifier, so the logic is to do a
> > kvm_put_kvm in the notifier too.
> > This is important because userland will ask us to release the KVM/VFIO
> > link through this notifier.
> > So I will have to rework this part where KVM==NULL in the notifier too.
> >
> > Regards,
> > Pierre
>
> I think it can be done both ways. If you ensure KVM != NULL if the open
> succeeds and take the reference in the notifier. I suppose if open()
> fails release() won't be called. But the logic/code in open() would get
> quite ugly because the callback could be called assync so that it
> overlaps with the rest of open().

Not necessary, but there is more than just the kvm_get_kvm().

When the user calls KVM_DEV_VFIO_GROUP_DEL he asks to break the link 
between VFIO and KVM.

Currently we just ignore this instead of stopping all activity 
associated with KVM.

But we have more bugs there:
We should not support multiple open of the mdev which will overwrite 
matrix->kvm for the same mdev with a different KVM.
I send a bugfix for this.


> Not failing open() in case of no KVM is there yet is in my opinion
> cleaner anyway.

If we handle correctly the notifiers and the exclusivity, we can do this.
I will make this correctly for the next iteration.

Regards,
Pierre

> Regards,
> Halil


--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany