On Wed, Apr 12, 2023 at 1:01 PM Parav Pandit <[email protected]> wrote:
>
>
>
> > From: Jason Wang <[email protected]>
> > 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
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