Re: [PATCH RFC] virtio-pci: support config layout in BAR1

[Rusty Russell <rusty@rustcorp.com.au>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> Some setups don't support enabling BAR0 (IO BAR). Reasons range from CPU
> limitations (e.g. on some powerpc setups) to architecture limmitations
> (e.g. a setup with >15 PCI bridges, with one virtio device behind each,
> on x86).
>
> PCI Express spec made IO optional, so future guests will disable IO for
> a device in more and more configurations.
>
> This patch makes it possible for host to mirror the config in BAR1, such
> that these setups can work properly.
>
> Guests with old drivers can't be fixed, they will continue to work as
> well (or as bad) as they did previously.  For this reason, changing
> revision id appears unnecessary - it would break setups that previously
> worked, partially.
>
> Future work on re-organizing layout won't conflict with this patch - it
> can use a different BAR or put config at an offset, or update revision.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Since QEMU won't support this, I think we'll have to wait for the PCI
capabilities, ie. the virtio spec 1.0.

Cheers,
Rusty.