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

[Tony Krowiak]

On 06/07/2018 01:15 PM, Pierre Morel wrote:
> > > We have internal structures like the ap_matrix and kvm_ap_matrix
> > > which look like the bus/devices we had previously but are differently
> > > or not at all integrated with the LDD.
> >
> > What is LDD? Are you talking about dependencies between the vfio_ap 
> > device
> > driver and KVM? If so, see my arguments below.
> >
> > > Also I think that with a little data structure refactoring you can 
> > > avoid most of
> > > the code in the arch/s390/kvm.
> >
> > How will structure refactoring help us avoid the code for updating 
> > the CRYCB
> > in the guest's SIE state description.
> >
> > > For example, storing the kvm pointer inside the kvm_ap_matrix and
> > > maintaining a list of the kvm_ap_matrix structures allows to easily 
> > > know
> > > if a guest already has an associated mediated device.
> >
> > How is that easier than storing the kvm pointer inside of the 
> > mediated matrix
> > device (i.e., struct ap_matrix_mdev) which also contains the struct 
> > kvm_ap_matrix?
>
> you can put it in ap_matrix_mdev but just the name "kvm_ap_matrix" 
> make the
> last one a better candidate for my opinion.

It's been in ap_matrix_mdev since v2, but I'll consider moving it to 
kvm_ap_matrix.

> > How does that allow us to avoid the code in arch/s390/kvm?
>
> This alone does not.
>
> > We still need the code
> > to update the CRYCB in the SIE block. I can obviously avoid placing 
> > that code in
> > kvm-ap.c and move it to the driver, but I already explained my 
> > reasoning for
> > keeping it in KVM. Let's face it, there is no way around the 
> > dependency between
> > the vfio_ap device driver and KVM unless guest matrix configuration 
> > is managed
> > solely by KVM through KVM interfaces.
>
> We get the pointer to KVM from the VFIO interface.
> That we both discuss on this is sterile.
> The only one who could say what is right is a S390 KVM maintainer.
> This would end the discussion.
> My point was just to say that we have an alternative.
>
>
> > Why maintain a list of kvm_ap_matrix structures if we don't have to; 
> > it is stored
> > with the mediated matrix device which is passed in to all of the 
> > vfio_ap driver
> > callbacks.
>
> Because using the vm_list which is a static in kvm makes you stick 
> inside the kvm code.

I understand your point here, but even if we did maintain a list of 
kvm_ap_matrix structures,
we still need the kvm code to configure the guest's CRYCB and eventually 
ECA.28. There is
also code in kvm-ap.c that is called from KVM. The idea behind kvm-ap.c 
is that all code
related to configuration of AP structures in KVM is in this one spot.

>