Re: [PATCH v5 11/13] KVM: s390: implement mediated device open callback

[Tony Krowiak]

On 06/05/2018 08:19 AM, Pierre Morel wrote:
> On 30/05/2018 16:33, Tony Krowiak wrote:
> > On 05/24/2018 05:08 AM, Pierre Morel wrote:
> > > On 23/05/2018 16:45, Tony Krowiak wrote:
> > > > On 05/16/2018 04:03 AM, Pierre Morel wrote:
> > > > > On 07/05/2018 17:11, Tony Krowiak wrote:
> > > > > > Implements the open callback on the mediated matrix device.
> > > > > > The function registers a group notifier to receive notification
> > > > > > of the VFIO_GROUP_NOTIFY_SET_KVM event. When notified,
> > > > > > the vfio_ap device driver will get access to the guest's
> > > > > > kvm structure. With access to this structure the driver will:
> > > > > >
> > > > > > 1. Ensure that only one mediated device is opened for the guest
> > >
> > > You should explain why.
> > >
> > > > > > 2. Configure access to the AP devices for the guest.
> > > ...snip...
> > > > > > +void kvm_ap_refcount_inc(struct kvm *kvm)
> > > > > > +{
> > > > > > +    atomic_inc(&kvm->arch.crypto.aprefs);
> > > > > > +}
> > > > > > +EXPORT_SYMBOL(kvm_ap_refcount_inc);
> > > > > > +
> > > > > > +void kvm_ap_refcount_dec(struct kvm *kvm)
> > > > > > +{
> > > > > > +    atomic_dec(&kvm->arch.crypto.aprefs);
> > > > > > +}
> > > > > > +EXPORT_SYMBOL(kvm_ap_refcount_dec);
> > > > >
> > > > > Why are these functions inside kvm-ap ?
> > > > > Will anyone use this outer of vfio-ap ?
> > > >
> > > > As I've stated before, I made the choice to contain all interfaces 
> > > > that
> > > > access KVM in kvm-ap because I don't think it is appropriate for 
> > > > the device
> > > > driver to have to have "knowledge" of the inner workings of KVM. 
> > > > Why does
> > > > it matter whether any entity outside of the vfio_ap device driver 
> > > > calls
> > > > these functions? I could ask a similar question if the interfaces were
> > > > contained in vfio-ap; what if another device driver needs access to 
> > > > these
> > > > interfaces?
> > >
> > > This is very driver specific and only used during initialization.
> > > It is not a common property of the cryptographic interface.
> > >
> > > I really think you should handle this inside the driver.
> >
> > We are going to have to agree to disagree on this one. Is it not 
> > possible
> > that future drivers - e.g., when full virtualization is implemented - 
> > will
> > require access to KVM?
>
> I do not think that an access to KVM is required for full virtualization.

You may be right, but at this point, there is no guarantee. I stand by my
design on this one.

>