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

Laine Stump laine at redhat.com
Thu Sep 30 13:22:18 UTC 2021


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outdated).

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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