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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 25, 2023 at 09:26:07AM -0300, Jason Gunthorpe wrote:
> > > So, as I keep saying, in this scenario the goal is no mediation in the
> > > hypervisor.
> > 
> > That's pretty fine, but I don't think trapping + relying is not
> > mediation. Does it really matter what happens after trapping?
> 
> It is not mediation in the sense that the kernel driver does not in
> any way make decisions on the behavior of the device. It simply
> transforms an IO operation into a device command and relays it to the
> device. The device still fully controls its own behavior.
> 
> 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?

> > > It is pointless, everything you think you need to do there
> > > is actually already being done in the DPU.
> > 
> > Well, migration or even Qemu could be offloaded to DPU as well. If
> > that's the direction that's pretty fine.
> 
> That's silly, of course qemu/kvm can't run in the DPU.
> 
> However, we can empty qemu and the hypervisor out so all it does is
> run kvm and run vfio. In this model the DPU does all the OVS, storage,
> "VPDA", etc. qemu is just a passive relay of the DPU PCI functions
> into VM's vPCI functions.
> 
> So, everything VDPA was doing in the environment is migrated into the
> DPU.
> 
> 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.
> 
> Jason

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. 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?


-- 
MST

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