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

Michael S. Tsirkin
2023-04-13T21:02:35Z

On Thu, Apr 13, 2023 at 01:24:24PM -0400, Parav Pandit wrote:
> > > > - work for virtualization (and it work even without dedicated
> > > > mediation for some setups)
> > > I am not aware where can it work without mediation. Do you know any
> > > specific kernel version where it actually works?
> >
> > E.g current Linux driver did:
> >
> > rc = pci_request_region(pci_dev, 0, "virtio-pci-legacy");
> >
> > It doesn't differ from I/O with memory. It means if you had a
> > "transitional" device with legacy MMIO BAR0, it just works.
> >
>
> Thanks to the abstract PCI API in Linux.

Right. I do however at least see the point of what Jason is proposing,
which is to enable some legacy guests without mediation in software.

This thing ... you move some code to the card and reduce the amount of
virtio knowledge in software but do not eliminate it completely.
Seems kind of pointless. Minimal hardware changes make more sense to
me, I'd say. Talking about that, what is a minimal hardware change
to allow a vdpa based solution?
I think that's VIRTIO_NET_F_LEGACY_HEADER, right?

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MST