Re: [Qemu-devel] RFC: Primary VGA

[Michael S. Tsirkin]

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?

> We'll also need some reasonable way
> to pick a default if unspecified.
> 
> Does anyone have any thoughts on managing this?  Thanks,
> 
> Alex