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