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