[PATCH v3 0/5] Add support for two i386 pm options which control acpi hotplug

[Ani Sinha]

On Thu, Sep 30, 2021 at 18:52 Laine Stump <laine at redhat.com> wrote:

> On 9/30/21 2:16 AM, Ani Sinha wrote:
> > On Fri, Sep 24, 2021 at 2:16 AM Laine Stump <laine at redhat.com> wrote:
> >>
> >> On 9/11/21 11:26 PM, Ani Sinha wrote:
> >>> The above two options are only available for qemu driver and that too
> for x86
> >>> guests only. Both of them are global options.
> >>>
> >>> ``acpi-hotplug-bridge`` option enables or disables ACPI hotplug
> support for cold
> >>> plugged bridges. Examples of cold plugged bridges include PCI-PCI
> bridge
> >>> (pci-bridge controller) for pc machines and pcie-root-port controller
> for q35
> >>> machines. The corresponding commandline options to qemu for x86 guests
> are:
> >>
> >> The "cold plugged bridges" term here throws me for a loop - it implies
> >> that hotplugging bridges is something that's supported, and I think it
> >> still isn't. Of course this is just the cover letter, so it won't go
> >> into git anywhere, but I think it should be enough to say "enables ACPI
> >> hotplug into non-root bus PCI bridges/ports".
> >
> > I think emphasizing cold plugged bridges is important as Igor (CC'd)
> > has clarified in the other email on patch #3 of this series.
>
> Okay, so the implication in Igor's email is that a) it is possible to
> hotplug a pcie controller, but b) any controller that is hotplugged will
> not have ACPI enabled. Note though that libvirt doesn't allow
> hotplugging *any* PCI controller, since we were told long ago that no OS
> will actually rescan the PCI topology when this is done, and so the new
> controller wouldn't be usable anyway. (that information may well be
> outdated).


>From i440fx side all empty ports in the pci root controller are described
as hotplug capable from ACPI. So I do not see why we cannot hotplug a pci
bridge in one of the pci root ports and OS should be able to detect it
without reboot. I have not tried it though.


>
> I think if you're going to mention that it is specifically for
> "cold-plugged bridges" then you should also 1) define what
> "cold-plugged" means, i.e. "(PCI controllers that were present in the
> domain definition when the guest was first started"), and 2) note that
> "ACPI is not enabled for bridges that are hot-plugged (but currently
> libvirt doesn't support hotplugging a pci controller anyway)" or
> something like that.
>
>
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