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