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

[Ani Sinha]

On Tue, Sep 28, 2021 at 10:17 PM Laine Stump <laine at redhat.com> wrote:

> 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


6.1 not only introduced this option/command line in Qemu but also flipped
the switch to make ACPI hotplug default for pcie root ports. So there are
 no officially released  version of Qemu where this command line exists and
the default is native pcie hotplug.


will simultaneously enable ACPI hotplug
> and disable native-pcie hotplug for all pcie-root-ports and
> pcie-downstream-ports.


This is for 6.1 based machines as well.

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 ).
>
>
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