[libvirt] [PATCH] qemu: qxl devices don't support multifunction yet

[Wen Congyang]

At 09/29/2011 10:56 PM, Laine Stump Write:
> On 09/29/2011 10:30 AM, Laine Stump wrote:
>> On 09/27/2011 05:13 AM, Daniel P. Berrange wrote:
>>> When we originally enabled multifunction for all PCI devices, we did so
>>> in the belief that this was effectively a no-op for guest OSes. ie it
>>> should not have changed guest OS behaviour at all, unless multiple
>>> devices
>>> were actually inserted in the slot.
>>>
>>> Evidentally this turns out to be incorrect. In other words we introduced
>>> a guest ABI change. This is very bad, because our stated goal is to have
>>> a stable guest ABI across libvirt&  QEMU releases.
>>>
>>> Thus IMHO we need to disable *all* setting of the 'multifunction=on'
>>> parameter for all guests by default, to unbreak the ABI compatibility.
>>>
>>> Then we should introduce a new parameter 'multifunction='on' in the
>>> <address type=pci>  element to allow it to be optionally enabled per
>>> device.
>>
>>
>> A couple of questions:
>>
>> 1) Is it possible/useful to have function > 0 without multifunction=on
>> in the qemu commandline?
>>
>> 2) If the answer to (1) is "No", if the XML has function > 0 but no
>> multifunction=on, should we implicitly add multifunction=on, or should
>> we log an error?
> 
> Also:
> 
> 3) if you explicitly/(implicitly?) set multifunction on for one function
> of a slot, does qemu automatically do so for other functions on the slot
> (ie, if you have a device that uses slot 1 function 1 (and turns on
> multifunction), then will slot 1 function 0 automatically have
> multifunction turned on too?)

IIRC, qemu does not do it. If bit 7 in the Header Type register of function
1 is 1, and bit 7 in the Header Type register of function 0 is 0, it's a single
device, and you will not see the other function on the guest.

Thanks
Wen Congyang

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