Re: [virtio-comment] Re: [PATCH 09/11] transport-pci: Describe PCI MMR dev config registers

[Jason Wang <jasowang@redhat.com>]

On Wed, Apr 12, 2023 at 1:01âPM Parav Pandit <parav@nvidia.com> wrote:
>
>
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Wednesday, April 12, 2023 12:51 AM
> >
> > Yes, but this proposal will drag us back to legacy, isn't it?
> No. This proposal supports the legacy transitional pci device.
>
> > Or if it is used only in
> > the vitalization environment, what's the advantages of doing it over simply
> > mediating on top of modern device?
> >
> Because the spec for modern device do not allow it. Discussed in these threads.

Can you please tell me which part of the spec disallows it? There's a
long discussion in the virtualization-list a few months ago about
mediating legacy devices on top of modern. We don't see any blocker do
you?

>
> > 1) legacy MMIO bar: spec changes, hypervisor mediation
> > 2) modern device: no spec changes, hypervisor mediation
> >
> This question repeats the same discussion occurred in this patch series.
> You might want to refer it again to avoid repeating all over again.

No, I think you miss the point that modern devices could be used for
mediating legacy devices.

>
> > 1) it's better not invent any of new facilities for legacy
> > 2) if legacy is insisted, allow MMIO BAR0 is much simpler and better
> You have missed few emails. :)
> MMIO BAR is proposed here and it is not limited to BAR 0.

In the context of mediation, why do you need that flexibility?

> It is left for the device to either map something in existing BAR or use BAR 0.
> Because PCI has only 3 BARs.
> A device may want to support legacy and non-legacy functionality both at the same time.

This is perfectly fine, this is how Qemu did:

    /*
     * virtio pci bar layout used by default.
     * subclasses can re-arrange things if needed.
     *
     *   region 0   --  virtio legacy io bar
     *   region 1   --  msi-x bar
     *   region 2   --  virtio modern io bar (off by default)
     *   region 4+5 --  virtio modern memory (64bit) bar
     *
     */

> So better not to hard wire for BAR 0.

Using BAR0 doesn't prevent you from adding modern capabilities in BAR0, no?

Thanks