[virtio-dev] Re: [PATCH 00/11] Introduce transitional mmr pci device

Michael S. Tsirkin
2023-04-03T15:07:05Z

On Mon, Apr 03, 2023 at 02:57:26PM +0000, Parav Pandit wrote:
>
>
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Monday, April 3, 2023 10:53 AM
> >
> > On Fri, Mar 31, 2023 at 05:43:11PM -0400, Parav Pandit wrote:
> > > > I can not say I thought about this
> > > > deeply so maybe there's some problem, or maybe it's a worse approach
> > > > - could you comment on this? It looks like this could be a smaller
> > > > change, but maybe it isn't? Did you consider this option?
> > >
> > > We can possibly let both the options open for device vendors to implement.
> > >
> > > Change wise transport VQ is fairly big addition for both hypervisor
> > > driver and also for the device.
> >
> > OTOH it is presumably required for scalability anyway, no?
> No.
> Most new generation SIOV and SR-IOV devices operate without any para-virtualization.

Don't see the connection to PV. You need an emulation layer in the host
if you want to run legacy guests. Looks like it could do transport vq
just as well.

> > And presumably it can all be done in firmware ...
> > Is there actual hardware that can't implement transport vq but is going to
> > implement the mmr spec?
> >
> Nvidia and Marvell DPUs implement MMR spec.

Hmm implement it in what sense exactly?

> Transport VQ has very high latency and DMA overheads for 2 to 4 bytes read/write.

How many of these 2 byte accesses trigger from a typical guest?

> And before discussing "why not that approach", lets finish reviewing "this approach" first.

That's a weird way to put it. We don't want so many ways to do legacy
if we can help it.

--
MST