Re: [PATCH v3 3/5] conf: introduce acpi-hotplug-bridge and acpi-root-hotplug pm options
On Tue, 28 Sep 2021, Laine Stump wrote:
On 9/28/21 12:54 PM, Ani Sinha wrote:
>
>
> On Tue, Sep 28, 2021 at 10:17 PM Laine Stump <laine(a)redhat.com
> <mailto:laine@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.
You're assuming that everyone will use the canonical "q35" machinetype
rather
than a specific versioned machinetype (e.g. "pc-q35-6.0"). For pre-6.1
machinetypes, the default will still be native-pcie hotplug, even when running
qemu-6.1+.
Yes you are right. If one uses the 6.1 binary but specifies a compat pre
6.1 machine type then yes, native hotplug would be the default and
acpi-hotplug-bridge will be off. If someone forces it on (not sure why
they would though because they could simply use the 6.1 machine), the acpi
hotplug would be enabled and native hotplug would get disabled on q35.
>
>
> 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 ).
>