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

and disable native-pcie hotplug for all pcie-root-ports and pcie-downstream-ports.

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 ).