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

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On Tue, Sep 28, 2021 at 3:28 PM Ani Sinha <ani@xxxxxxxxxxx> wrote:
>
>
>
> 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

https://patchwork.kernel.org/project/qemu-devel/patch/1478099802-14188-1-git-send-email-marcel@xxxxxxxxxx
has some discussions around shpc.

Also
"PCI devices can be hot-plugged into PCI Express to PCI and PCI-PCI Bridges.
The PCI hot-plug into PCI-PCI bridge is ACPI based, whereas hot-plug into
PCI Express to PCI bridges is SHPC-based. They both can work side by side with
the PCI Express native hot-plug."

https://android.googlesource.com/platform/external/qemu/+/emu-master-dev/docs/pcie.txt

My own experiments on i440fx has shown that Windows does not seem to
care about shpc.
Disabling acpi hotplug on bridges disables hotplug.

> >
> > >
> > > >  - 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
> > --
> > |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> > |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> > |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
> >
> >





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