On Thu, 2013-02-28 at 08:50 +0200, Michael S. Tsirkin wrote: > On Wed, Feb 27, 2013 at 02:16:23PM -0700, Alex Williamson wrote: > > > > When we start adding root ports and bridges to systems we need some > > concept of a primary VGA device.The differentiation of the primary > > device is that it's the default one that responds to the Legacy VGA > > address ranges. PCs often have a BIOS selection for this. > > > > Seabios already seems to have some concept of this and looks for a VGA > > class device for which the parent devices all have VGA routing enabled. > > This seems to work today if QEMU initializes VGA routing for the path it > > considers the primary. > > > > The first question is whether this bridge path pre-configuration is what > > we want to keep as the way QEMU communicates the primary VGA device to > > Seabios? Obviously we could switch to some kind of fwcfg interface, but > > I tend to think what we have is sufficient. > > > > If it is sufficient, then I think we need to rebuild that path on system > > reset and we need some way to specify which device to use. One option > > would be some kind of per PCIDevice property, perhaps "primary_vga". A > > downside is that users can abuse it by trying to set it for more than > > one device. Maybe a better approach would be to add a machine property > > for it, -machine primary_vga=$id. > > Yes. And a command to change it when we support hotplug in the future?
Looking at how this would happen on bare metal, there are ACPI methods _GPD (Get Post Device), _SPD (Set Post Device), and _VPD (Video Post Options). So I imagine that if we supported VGA hotplug we'd use those to let the guest specify the primary and continue to use an algorithm in SeaBIOS to post to the lowest B:D.F VGA device if unspecified. I don't know that we need some kind of QMP/QAPI runtime command to change the VGA post device externally on the next boot, real systems likely doesn't have such a hook. For now I'll just start with making SeaBIOS have a way to enable and post the first VGA device when it's behind bridges. Thanks, Alex > > We'll also need some reasonable way > > to pick a default if unspecified. > > > > Does anyone have any thoughts on managing this? Thanks, > > > > Alex
