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

Parav Pandit
2023-04-03T14:57:31Z

> 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.

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

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