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

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Wednesday, April 12, 2023 1:15 AM

> > 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?
>
Modern device says FEAETURE_1 must be offered and must be negotiated by driver.
Legacy has Mac as RW area. (hypervisor can do it).
Reset flow is difference between the legacy and modern.

> >
> > > 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?
> 
The PCI device exposed is transitional to the guest VM, so it can do legacy as well as newer features.
And if BAR 0 is hard coded, it may not be able to support features that may need additional BAR.

> > 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?
>
Right, it doesnât. But spec shouldnât write BAR0 is only for legacy MMIO emulation, that would prevent BAR0 usage.