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

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On Mon, Sep 25, 2023 at 8:26 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> On Mon, Sep 25, 2023 at 10:34:54AM +0800, Jason Wang wrote:
> > > Cloud vendors will similarly use DPUs to create a PCI functions that
> > > meet the cloud vendor's internal specification.
> >
> > This can only work if:
> >
> > 1) the internal specification has finer garin than virtio spec
> > 2) so it can define what is not implemented in the virtio spec (like
> > migration and compatibility)
> Yes, and that is what is happening. Realistically the "spec" isjust a
> piece of software that the Cloud vendor owns which is simply ported to
> multiple DPU vendors.
> It is the same as VDPA. If VDPA can make multiple NIC vendors
> consistent then why do you have a hard time believing we can do the
> same thing just on the ARM side of a DPU?

I don't. We all know vDPA can do more than virtio.

> > All of the above doesn't seem to be possible or realistic now, and it
> > actually has a risk to be not compatible with virtio spec. In the
> > future when virtio has live migration supported, they want to be able
> > to migrate between virtio and vDPA.
> Well, that is for the spec to design.

Right, so if we'd consider migration from virtio to vDPA, it needs to
be designed in a way that allows more involvement from hypervisor
other than coupling it with a specific interface (like admin

> > > 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.
> > > 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.

KVM can't for sure but part of Qemu could. This model has been used.

> 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.

It really depends on the use cases. For example, in the case of DPU
what if we want to provide multiple virtio devices through a single

> 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.

That's pretty fine.


> Jason

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