[PATCH v3 3/5] conf: introduce acpi-hotplug-bridge and acpi-root-hotplug pm options

[Laine Stump]

On 9/28/21 4:44 AM, Daniel P. Berrangé wrote:
> On Sun, Sep 12, 2021 at 08:56:29AM +0530, Ani Sinha wrote:
>> This change introduces libvirt xml support for the following two pm options:
>>
>> <pm>
>>    <acpi-hotplug-bridge enabled='no'/>
>>    <acpi-root-hotplug enabled='yes'/>
>> </pm>
> 
> 
>> +``acpi-hotplug-bridge``
>> +   :since:`Since 7.8.0` This option enables or disables BIOS ACPI based hotplug support
>> +   for cold plugged bridges. It is available only for x86 guests, both for q35 and pc
>> +   machine types. For pc machines, the support is available from `QEMU 2.12`. For q35
>> +   machines, the support is available from `QEMU 6.1`. Examples of cold plugged bridges
>> +   include PCI-PCI bridges for pc machine types (pci-bridge controller). For q35 machines,
>> +   it includes PCIE root ports (pcie-root-port controller). This is a global option that
>> +   affects all bridges. No other bridge specific option is required to be specified.
> 
> Can you confirm my understanding of the situation..
> 
>   - i440fx / PCI topology - hotplug always uses ACPI
> 
>   - q35 / PCIe topology - hotplug historically used native PCIe hotplug,
>                           but in 6.1 switched to ACPI
> 
> Given, the name "acpi-hotplug-bridge",  am I right that this option
> has *no* effect, if the q35 machine is using native PCIe hotplug
> approach ?  IOW, is it a no-op until 6.1 based machine types for q35 ?

I *think* that in machinetypes where the default is native-pcie hotplug, 
setting acpi-hotplug-bridge=on will simultaneously enable ACPI hotplug 
and disable native-pcie hotplug for all pcie-root-ports and 
pcie-downstream-ports. Similarly on 6.1-based machinetypes, setting 
acpi-hotplug-bridge=off will disable ACPI hotplug and enable native-pcie 
hotplug.

On 440fx, where the default has always been ACPI (and where SHPC hotplug 
has been disabled), acpi-hotplug-bridge=on will be a NOP, and 
acpi-hotplug-bridge=off will completely disable hotplug on any 
pci-bridge (but *not* on pci-root).

As for the acpi-hotplug-root option, that is only valid for 440fx, is 
"on" by default, and when acpi-hotplug-root=off it will completely 
disable hotplug to any slot on pci-root.

(for completeness - when a pcie-root-port or pcie-downstream-port has 
<target hotplug='off'/>, that will disable whatever hotplug mode would 
have been enabled for the controller - no hotplug at all will be 
possible on that controller. QEMU also has a "native_hotplug" option 
(not supported in libvirt) for pcie-root-ports and pcie-downstream-ports 
which can be used to enable native-pcie hotplug on a specific controller 
when it had been disabled (in favor of ACPI) with the global 
acpi-hotplug-bridge ).