Re: [Qemu-devel] [PATCH] virtio-pci: make pci bar layout more flexible.

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

On Wed, Mar 04, 2015 at 12:51:35PM +0100, Michael S. Tsirkin wrote:
> On Wed, Mar 04, 2015 at 12:46:50PM +0100, Gerd Hoffmann wrote:
> > On Mi, 2015-03-04 at 12:04 +0100, Michael S. Tsirkin wrote:
> > > On Wed, Mar 04, 2015 at 11:55:50AM +0100, Gerd Hoffmann wrote:
> > > >   Hi,
> > > > 
> > > > > > VirtIOPCIProxy subclasses which need additional pci bars, such as
> > > > > > virtio-vga, just need to make sure they register the additinal bars
> > > > > > before initializing virtio-pci, so the superclass can see the registered
> > > > > > bars and shuffle around the virtio bars accordingly.
> > > > > 
> > > > > I think I prefer we just DTRT and keep same layouts for everyone by
> > > > > default: isn't there a layout that is good for everybody?
> > > > 
> > > > I want bar #2 for the vga framebuffer for virtio-vga.  Which conflicts
> > > > with bar #2 being used for the modern bar in todays code.
> > > 
> > > OK, and IIUC you are saying that using bar #2 for that is a requirement for
> > > vga to work?
> > 
> > Makes things alot easier as seavgabios has support for virtio-vga (in
> > vga compat mode) already and expects the framebuffer being in bar #2.
> > 
> > It's not a _really_ hard requirement though.
> > 
> > cheers,
> >   Gerd
> > 
> 
> Easy is good, I think we don't really care where the modern bar is,
> let's just move it for everyone, and add a comment explaining the
> motivation.

OTOH if we really want to make it flexible, we can add a new
virtio pci capability, with a new cfg_type value, say 0x6,
and use that to describe BAR and offset for vga use.
I'm not sure I understand all the implications though.

> -- 
> MST