On Tue, Sep 28, 2021 at 11:47:26AM +0100, Daniel P. Berrangé wrote:
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.
Opps, I was mislead. They have shpc=false by default due to machine types >= 2.9 overriding it to false
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 ?
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 :|