"Michael S. Tsirkin" [off-list ref] 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.