[virtio-dev] RE: [PATCH 08/11] transport-pci: Introduce virtio extended capability

Parav Pandit
2023-05-21T13:25:03Z

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Sunday, May 21, 2023 1:58 AM

> Yes but neither can it support new devices because it does not look for the new
> device id. So it's fine - I was talking about new devices using extended
> capability.
>
> We shouldn't just laser-focus on existing software and devices, there's probably
> more to come.
>
Sure, new devices can use new capability.
I will roll this as separate patch and github issue.
As we discussed, legacy register access over AQ does not need this capability anyway.

So not related to v2 discussion.
I am in middle of splitting this as separate patch unrelated to the v2 for legacy access.

>
> > But that is the case with any existing software, not just seabios.
> >
> > New capability is in addition to the existing one.
> > And code already exists to refer to the old one, so I am not seeing any gain to
> create them now.
> > Any new capability that one creates in the future can be in the extended area.
>
> Question is about MCFG in general. Is adding capability to access MCFG to
> seabios for pci accesses practical? hard?
>
What is MCFG?

> > You mentioned a perf angle that it takes half the time, but I don't
> > see how as the only change between legacy and ext is: offset.
>
> For a variety of reasons Linux does not use memory mapped accesses for legacy
> config space, it uses cf8/cfc for that.
And does it use memory mapped accesses for non-legacy config space?