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"
> 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 ?
> 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)
* 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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