Re: [Qemu-devel] default slot used for vga device on q35 machines

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

>>> 1) Is this difference intentional, or a bug?
>> The vga simply goes into the first free slot.  That happens to be #2
>> with i440fx and #1 with q35.
> 
> It sounds like it's a safe bet to assume that -vga will put the device
> on slot 2 for pc machinetypes and slot 1 for q35, no matter what other
> devices there are, since -vga is always initialized first. Correct?

Yes.

> Do
> you see any chance that might change in the future? (e.g. due to some
> other new device that needs to be initialized even before vga)

Highly unlikely.  Also libvirt will switch over to use -device for vga
cards, which will make this a moot point.

>>> 3) Does the qxl multihead support really require that the device be at
>>> slot 2 (as stated in the above bugzilla commend)? Or is that just a
>>> misunderstanding/overstatement?
>> It's not required at all.  The problem is that on older qemu versions
>> (pre-memory-api basically) it was impossible to create functional vga
>> devices via -device due to an initialization order issue.  Which implies
>> you have to use -vga instead, which in turn implies the vga ends up in
>> slot #2.  On i440fx, but q35 didn't exist yet back then ;)
> 
> Are you saying that it's still required to use -vga instead of -device
> qxl-vga for pc machinetype? Or that it was needed at one time, but that
> is no longer the case? If the latter, is there a reliable way to make
> the decision whether or not we need to use -vga?

libvirt has started using -device for vga cards, to be able to place
them in any pci slot.  IIRC it's used for qemu 1.1+ or 1.2+.  That works
fine for all vga cards except qxl.  The bug which broke it for qxl will
be fixed in 1.6 (and probably 1.5.3 too).

cheers,
  Gerd