Re: [PATCH v3 3/5] conf: introduce acpi-hotplug-bridge and acpi-root-hotplug pm options
On Tue, 28 Sep 2021, 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
ACPI is the primary means of enabling hotplug. shpc might also have a role
here but I think it is disabled. Igor (cc'd) might throw some lights on
how shpc comes to play.
- q35 / PCIe topology - hotplug historically used native PCIe
hotplug,
but in 6.1 switched to ACPI
Correct.
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 ?
Its complicated.
With "acpi-hotplug-bridge" ON, native hotplug is disabled in qemu.
With "acpi-hotplug-bridge" OFF, native hotplug is enabled in qemu.
Libvirt does not allow it, but by directly using qemu commandline it is
possible to enable ACPI hotplug for pcie-root-ports by turning
"acpi-hotplug-bridge" ON _and_ enable native hotplug at the same time. In
that case, as Julia has mentioned in some other thread, the guest OS
always choses native over ACPI based hotplug.
The <target hotplug="on/off"> switch in libvirt for pcie-root-ports
currently does not care whether native or acpi hotplug is used. It simply
turns on the hotplug for that particular port. Whether ACPI or native is
used is controlled by this global flag that Julia has introduced in 6.1.
IOW, is it a no-op until 6.1 based machine types for q35 ?
Correct. For q35 this command line did not exist until 6.1.