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

Ani Sinha ani at anisinha.ca
Tue Sep 28 09:58:04 UTC 2021 


On Tue, 28 Sep 2021, Daniel P. Berrangé wrote:

> On Tue, Sep 28, 2021 at 02:35:47PM +0530, Ani Sinha wrote:
> >
> >
> > 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.
>
> Yes, I think it will be important to understand if 'shpc' becomes relevant
> when ACPI hotplug is disabled for PCI
>
> >
> > >  - 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.
>
> Oh, I mis-read and didn't realize this was controlling the QEMU
> "acpi-pci-hotplug-with-bridge-support" configuration.
>
> With this in mind I think the naming is somewhat misleading. Setting it
> to off would give users the impression that hotplug is disabled, which
> is not the case for Q35 at least. It is just switching to a different
> hotplug implementation.
>
> At least from Q35 pov, I think it would be better to call it
>
>     hotplug-mode="acpi|pcie"
>
> so it is clear that no matter what value it is set to, hotplug
> is still available.
>
> If we also consider PIIX, then depending on the answer wrt shpc
> above, we might want one of
>
>     hotplug-mode="acpi|pcie|none"
>     hotplug-mode="acpi|pcie|shpc"
>

If libvirt does not deal with shpc today I think we should not bother with
shpc at all. We should simply have a boolean mode appropriately named that
choses between acpi hotplug vs native.

>
> > 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.
>
> I see that is refering to pie-root-port gaining  native_hotplug=bool in 6.1
>
> You say the guest OS always chooses native over ACPI - is that reqiired
> in the spec, or is that just how some common guest OS have choosen to
> impl it today ?
>

I am not sure but I *think* this is guest OS dependent too? Julia to
rescue here :-)

>
> > 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.
>
> Right so we have
>
>   * PIIX4
>
>       - acpi-root-pci-hotplug=bool
>
>         Whether hotplug is enabled for the root bridge or not
>
>           <target hotplug="on/off"> for pci-root controller
>
>
>       - acpi-pci-hotplug-with-bridge-support=bool
>
>         Toggles support for ACPI based hotplug across all bridges.
> 	If disabled will there will be no hotplug at all for PIIX4 ?
> 	Or does 'shpc' come into play in that scenario ?
>
>
>    PIIX combinations
>
>        (1) acpi-root-pci-hotplug=yes
>            acpi-pci-hotplug-with-bridge-support=yes
>
>          - All bridges have hotplug
>
>        (2) acpi-root-pci-hotplug=yes
>            acpi-pci-hotplug-with-bridge-support=no
>
>          - No bridges have hotplug
>
>        (3) acpi-root-pci-hotplug=no
>            acpi-pci-hotplug-with-bridge-support=yes
>
>          - All bridges except root have hotplug
>
>        (4) acpi-root-pci-hotplug=no
>            acpi-pci-hotplug-with-bridge-support=no
>
>          - No bridges have hotplug. Essentially identical to (2)

no (4) is not identical to (2). In (4) no hotplug is enabled. In (2) pci
root bus still has hotplug enabled.


>
>   * Q35
>
>
>       - acpi-pci-hotplug-with-bridge-support=bool
>
>         Toggles support for ACPI based hotplug. If disabled native
> 	PCIe hotplug is activated instead
>
>
>   * pcie-root-port
>
>       - hotplug=bool
>
>         Toggles whether hotplug is enabled on the port. Affects
> 	either native and ACPI based hotplug, depending on what
> 	acpi-pci-hotplug-with-bridge-support=bool is set to ?
>
>           <target hotplug="on/off"> for pcie-root-port controller
>
>       - native_hotplug=bool
>
>         Can be used to also enable native hotplug, even when ACPI
> 	based hotplug is globally enabled. IOW only really relevant
> 	when acpi-pci-hotplug-with-bridge-support=true
>
>
>    Q35 combinations
>
>        (1) acpi-pci-hotplug-with-bridge-support=yes
>            pcie-root-port.hotplug=yes
>            pcie-root-port.native_hotplug=yes
>
>            ports have both ACPI + native hotplug, guest prefers native
>
>
>        (2) acpi-pci-hotplug-with-bridge-support=yes
>            pcie-root-port.hotplug=no
>            pcie-root-port.native_hotplug=yes
>
>            ports have no hotplug, despite native_hotplug=yes IIUC
>
>
>        (3) acpi-pci-hotplug-with-bridge-support=yes
>            pcie-root-port.hotplug=yes
>            pcie-root-port.native_hotplug=no
>
>            ports have ACPI hotplug only
>
>
>        (4) acpi-pci-hotplug-with-bridge-support=yes
>            pcie-root-port.hotplug=no
>            pcie-root-port.native_hotplug=no
>
>            ports have no hotplug
>
>
>        (5) acpi-pci-hotplug-with-bridge-support=no
>            pcie-root-port.hotplug=yes
>            pcie-root-port.native_hotplug=yes
>
>            ports have native hotplug
>
>
>        (6) acpi-pci-hotplug-with-bridge-support=no
>            pcie-root-port.hotplug=no
>            pcie-root-port.native_hotplug=yes
>
>            ports have no hotplug, despite native_hotplug=yes IIUC
>
>
>        (7) acpi-pci-hotplug-with-bridge-support=no
>            pcie-root-port.hotplug=yes
>            pcie-root-port.native_hotplug=no
>
>            ports have no hotplug, despite hotplug=yes IIUC
>
>
>        (8) acpi-pci-hotplug-with-bridge-support=no
>            pcie-root-port.hotplug=no
>            pcie-root-port.native_hotplug=no
>
>            ports have no hotplug
>
>
>
> Regards,
> Daniel
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