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

On 08/03/2013 01:32 PM, Laine Stump wrote:

>>> 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? 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)

Any future qemu will be new enough to not require the use of -vga, so we
only have to worry about the behavior of 1.5 and earlier with regards to
placement of -vga.

>>> 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 ;)
> 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?

Regarding the decision on whether to use -device or -vga, our hands are
tied.  With qemu 1.5 or earlier, we HAVE to use -vga, where spice
initialization had a bug such that use of -device would not display the
mouse.  That bug has been fixed for qemu 1.6, so for qemu 1.6, we prefer
to use -device.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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