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

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

>   qemu-kvm -M q35 -nodefaults -nodefconfig -qmp unix:/tmp/qemu,server
> -vnc :15 -vga std -usb
> 
> Then ran "query-pci" in the qmp monitor and found that the vga device is
> put at slot 1 instead of slot 2.
> 
> My questions:
> 
> 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.

> 2) If it's intentional, will the device always be at slot 1 (and trigger
> an error if something else is also placed at slot 1), or is it just
> picking the first unused slot? (that would *not* be good, because we
> must be able to predict what device is in which slot and prevent them
> from changing from run to run).

The vga is the very first (pci) device being created (when selecting via
-vga), so it gets the slots mentioned above no matter what, even though
simply picking the first unused slot.

> 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 ;)

cheers,
  Gerd