RE: [virtio-comment] Re: [PATCH V2 4/6] virtio-pci: implement VIRTIO_F_QUEUE_STATE

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, November 22, 2023 12:45 PM
> 
> On Wed, Nov 22, 2023 at 12:15:44PM +0800, Jason Wang wrote:
> > On Wed, Nov 22, 2023 at 12:26âAM Parav Pandit <parav@nvidia.com>
> wrote:
> > >
> > >
> > > > From: Jason Wang <jasowang@redhat.com>
> > > > Sent: Tuesday, November 21, 2023 10:01 AM
> > > >
> > > > On Fri, Nov 17, 2023 at 6:06âPM Parav Pandit <parav@nvidia.com>
> wrote:
> > > > >
> > > > >
> >
> > [...]
> >
> > > > > Sorry, no virtio specification does not support device migration today.
> > > > > Nothing is broken by adding new features.
> > > > >
> > > > > Above [1] has the right proposal that Jason's paper pointed out.
> > > > > Please use
> > > > it.
> > > >
> > > > I was involved in the design in [1]. And I don't see a connection
> > > > to the dicussion here
> > > >
> > > > 1) It is based on vDPA in L0
> > > > 2) It doesn't address the nesting issue, it requires a proper
> > > > design in the virtio spec to support migration in the nesting layer.
> > >
> > > Nothing prevents [1] to be done without vdpa.
> >
> > Well, Qemu has SR-IOV emulation for IGB. What's the point of the above
> > reply? If a hypervisor wishes, it can be done with any device on L0.
> >
> > We discuss the possibility of migrating nested VMs, not the
> > possibility of emulating SR-IOV, no?
> >
> > Thanks
> 
> It depends really. If the way to support nesting is to emulate sr-iov then the
> question becomes how hard it is to emulate sriov to the point where nesting
> works.
> 
> The fact that current hypervisors pretend that a VF is a PF when passing things
> on to guest and things still seem to work is nice but it's not a hugely important
> design point IMHO.

Nesting is not a virtio problem as if nested PCI devices needed, industry will work on it anyway that virtio will be able to leverage.
We should focus on two practical and immediate cases of vfio and vdpa that for first level guest.
And admin command + admin queue on the owner device seems to suffice both the use cases.
We should progress towards this and unblock users to start using it.