[libvirt] RFC: revival of hotplug/unplug for PCI Multifunction devices in QEMU guests

[Daniel Henrique Barboza]

On 6/19/19 4:31 AM, Daniel P. Berrangé wrote:
> On Tue, Jun 18, 2019 at 03:04:40PM -0300, Daniel Henrique Barboza wrote:
>> Hi,
>>
>> This is labeled as RFC but it's more like a FYI to let people know and
>> comment beforehand. Shiva sent a 28 patch series last year that implements
>> hotplug/unplug support for PCI multifunction devices [1]. The design
>> motivation of his work was based in a RFC sent to this mailing list back
>> in 2016 [2].
>>
>> I'll briefly summarize the goals and motivations here. What we have today
>> in Libvirt:
>>
>> - no hotplug/unplug support for multifunction PCI devices
>>
>> This is explained in details in [2]. When hotplugging a multifunction
>> device, QEMU will queue the hotplug operation of all non-zero functions and,
>> when function 0 is hotplugged, all functions are hotplugged together. This
>> is true for all archs that supports PCI multifunction devices in QEMU. For
>> unplug it varies: x86 will unplug all functions if any function is
>> unplugged, ppc64 needs to unplug each one.
> Do you know anything about why ppc64 & x86 are different in this respect
> in QEMU.  I think it would be desirable to fix QEMU so that unplug works
> consistently across architectures. These kind of behavioural differences
> are a cause of pain as x86 gets all the day to day testing & leaving
> ppc64 to bitrot if it behaves differently.

That's a good point. I don't know why it's different but I'll 
investigate it.


Thanks,

DHB


>
> Regards,
> Daniel