Re: [PATCH] virtio-vga: fix virtio-vga bar ordering

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 22, 2020 at 12:49:41PM +0200, Gerd Hoffmann wrote:
> On Wed, Apr 22, 2020 at 02:04:36AM -0400, Michael S. Tsirkin wrote:
> > On Tue, Apr 21, 2020 at 11:48:53PM +0200, Anthoine Bourgeois wrote:
> > > With virtio-vga, pci bar are reordered. Bar #2 is used for compatibility
> > > with stdvga. By default, bar #2 is used by virtio modern io bar.
> > > This bar is the last one introduce in the virtio pci bar layout and it's
> > > crushed by the virtio-vga reordering. So virtio-vga and
> > > modern-pio-notify are incompatible because virtio-vga failed to
> > > initialize with this option.
> > > 
> > > This fix exchange the modern io bar with the modern memory bar,
> > > replacing the msix bar that is never impacted anyway.
> > > 
> > > Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
> > 
> > Such changes generally need to be tied to machine version.
> 
> Given that modern-pio-notify is off by default and
> virtio-vga,modern-pio-notify=on is broken I think we don't need to worry
> about compatibility in this specific case (assuming the patch is updated
> to not shuffle around the msix bar, see other reply).
> 
> cheers,
>   Gerd

OK, just worth documenting that this will break cross-version
migration if modern-pio-notify is enabled.