5:47 a.m.
On Tue, Sep 28, 2021 at 03:28:04PM +0530, Ani Sinha 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
>
> >
> > > - 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.
I want to understand what's possible at the qemu hardware level,
so we don't paint ourselves into a corner.
IIUC, with shpc we only have a toggle on "pci-bridge" devices,
and those currently have shpc=true by default. There's no shpc
setting on the pci-root, and theres no global setting.
Seems to imply that if we have acpi-hotplug disabled for PIIX,
then there would be no hotplug on the pci-root, but shpc hotplug
would still be available on any pci-bridge devices ?
> > 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.
So you're saying that acpi-root-pci-hotplug=yes overrides the
global request acpi-pci-hotplug-with-bridge-support=no and
turns ACPI hotplug back on for the pcie-root
> * 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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>
>
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 :|