Re: [PATCH vfio 11/11] vfio/virtio: Introduce a vfio driver over virtio devices

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On Mon, Sep 25, 2023 at 09:40:59PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 25, 2023 at 03:44:11PM -0400, Michael S. Tsirkin wrote:
> > > VDPA is very different from this. You might call them both mediation,
> > > sure, but then you need another word to describe the additional
> > > changes VPDA is doing.
> > 
> > Sorry about hijacking the thread a little bit, but could you
> > call out some of the changes that are the most problematic
> > for you?
> I don't really know these details. The operators have an existing
> virtio world that is ABI toward the VM for them, and they do not want
> *anything* to change. The VM should be unware if the virtio device is
> created by old hypervisor software or new DPU software. It presents
> exactly the same ABI.
> So the challenge really is to convince that VDPA delivers that, and
> frankly, I don't think it does. ABI toward the VM is very important
> here.

And to complete the picture, it is the DPU software/firmware that
is resposible for maintaining this ABI in your ideal world?

> > > In this model the DPU is an extension of the hypervisor/qemu
> > > environment and we shift code from x86 side to arm side to increase
> > > security, save power and increase total system performance.
> > 
> > I think I begin to understand. On the DPU you have some virtio
> > devices but also some non-virtio devices.  So you have to
> > use VFIO to talk to the DPU. Reusing VFIO to talk to virtio
> > devices too, simplifies things for you. 
> Yes
> > If guests will see vendor-specific devices from the DPU anyway, it
> > will be impossible to migrate such guests away from the DPU so the
> > cross-vendor migration capability is less important in this
> > use-case.  Is this a good summary?
> Well, sort of. As I said before, the vendor here is the cloud
> operator, not the DPU supplier. The guest will see an AWS virtio-net
> function, for example.
> The operator will ensure that all their different implementations of
> this function will interwork for migration.
> So within the closed world of a single operator live migration will
> work just fine.
> Since the hypervisor's controlled by the operator only migrate within
> the operators own environment anyhow, it is an already solved problem.
> Jason

Okay the picture emerges I think. Thanks! I'll try to summarize later
for everyone's benefit.


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