[virtio-dev] Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ

Michael S. Tsirkin
2023-05-10T07:43:30Z

On Wed, May 10, 2023 at 03:01:25PM +0800, Jason Wang wrote:
> On Wed, May 10, 2023 at 2:05 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, May 08, 2023 at 10:23:39AM +0800, Jason Wang wrote:
> > > > I thought so too originally. Unfortunately I now think that no, legacy is not
> > > > going to be a byproduct of transport virtqueue for modern -
> > > > it is different enough that it needs dedicated commands.
> > >
> > > If you mean the transport virtqueue, I think some dedicated commands
> > > for legacy are needed. Then it would be a transport that supports
> > > transitional devices. It would be much better than having commands for
> > > a partial transport like this patch did.
> >
> > OK I am beginning to get what you are saying. So your criticism is
> > this: what if device supports vq transport for modern, and we want to
> > build a transitional device on top. how will that look. yes?
>
> Yes. I think it needs to be done through the transport virtqueue
> otherwise the transport is not self-contained.

I mean, any feature can be done over transport vq.

But there is value in adding legacy commands to an otherwise
modern device without reworking it completely to
switch to a different transport.

> > A reasonable thing to include at least in the commit log. Parav?
> >
> > You are also asking what if the device uses transport vq,
> > and we want transitional on top of that.
> > It's a fair question but I don't exactly get why would
> > this legacy support feature be wanted for the vq transport
> > and not for other transports.
>
> Not sure I get the question, but all the existing transport support
> legacy, if we want to have another, should the legacy support be a
> must or not?

This specific proposal is for tunneling legacy over admin vq.
It can live alongside a normal modern VF, with hypervisor
combining these to create a transitional device.

> >
> >
> >
> >
> > > > Consider simplest case, multibyte fields. Legacy needs multibyte write,
> > > > modern does not even need multibyte read.
> > >
> > > I'm not sure I will get here,
> >
> > What does this mean?
>
> I meant I don't get what the issue if "modern does not even need
> multibyte read".

parse error again. reword?

> >
> > > since we can't expose admin vq to
> > > guests, it means we need some software mediation. So if we just
> > > implement what PCI allows us, then everything would be fine (even if
> > > some method is not used).
> > >
> > > Thanks
> >
> > To repeat discussion on one of the previous versions, no it will not be
> > fine because legacy virtio abuses pci in fundamentally broken ways.
> > So yes you need a mediator on the host but even giving this
> > mediator a chance to be robust on top of hardware
> > means the hardware interface can not simply mirror legacy
> > to hardware.
> >
> > For example, host mediator needs to trap writes into mac,
> > buffer them and then send a 6 byte write.
> > Now, pci actually does allow 6 byte writes, but legacy
> > guests instead to 6 single byte writes for portability reasons.
>
> It's a known race condition, so PCI over adminq doesn't make it worse.

it can however make it better - you can do a single 6 byte write command.

> The mediator can just mirror what guests write over the admin
> commands.

and this "just" just isn't good enough, or we end up with hacks
in hardware.

> Thanks
>
> > --
> > MSr