Re: [PATCH v3 0/5] Add support for two i386 pm options which control acpi hotplug

Hi all: This patchset introduces libvirt xml support for the following two pm conf options: <pm> <acpi-hotplug-bridge enabled='no'/> <acpi-root-hotplug enabled='yes'/> </pm>

The above two options are only available for qemu driver and that too for x86 guests only. Both of them are global options. ``acpi-hotplug-bridge`` option enables or disables ACPI hotplug support for cold plugged bridges. Examples of cold plugged bridges include PCI-PCI bridge (pci-bridge controller) for pc machines and pcie-root-port controller for q35 machines. The corresponding commandline options to qemu for x86 guests are: (pc machines): -global PIIX4_PM.acpi-pci-hotplug-with-bridge-support=<off/on> (q35 machines): -global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=<off/on> Being global options, no other bridge specific options for pci-bridge controller or pcie-root-port controllers are required. For pc machine type in x86, this option is available in qemu for a long time, from version 2.1. Please see the changes in qemu.git: 9e047b982452c6 ("piix4: add acpi pci hotplug support") 133a2da488062e ("pc: acpi: generate AML only for PCI0 devices if PCI bridge hotplug is disabled") For q35 machine type, this was introduced in qemu 6.1 with the following changes in qemu.git: (a) c0e427d6eb5fef ("hw/acpi/ich9: Enable ACPI PCI hot-plug") (b) 17858a16950860 ("hw/acpi/ich9: Set ACPI PCI hot-plug as default on Q35") The reasons for enabling ACPI based hotplug for PCIe (q35) based machines (as opposed to native hotplug) for bridges are outlined in (b). It is possible that some users might still want to use native hotplug on PCIe [1]. Therefore, this conf option enables users to choose either ACPI based hotplug or native hotplug for cold plugged bridges (for example for pcie root port controller in q35 machines). ``acpi-root-hotplug`` option enables or disables ACPI based hotplug for PCI root bus (pci-root controller). This option is only available for pc machine type. The corresponding commandline option to qemu for x86 guests is: -global PIIX4_PM.acpi-root-pci-hotplug=<off/on> This additional option enables users to disable hotplug for all devices in the system without adding an additional PCI-PCI bridge, putting the devices behind the bridge and using the existing ``acpi-hotplug-bridge`` option to disable hotplug on that bridge. This feature was introduced from qemu version 5.2 with the following change in qemu.git: 3d7e78aa7777f ("Introduce a new flag for i440fx to disable PCI hotplug on the root bus") The above qemu commit describes some compelling reasons why users might to disable hotplug on PCI root buses [2]. A brief summary of the patches: >[PATCH v3 1/5] qemu: capablities: detect presence of >[PATCH v3 2/5] qemu: capablities: detect presence of Patches 1 and 2 implement support for qemu capability checks for the above config options. >[PATCH v3 3/5] conf: introduce acpi-hotplug-bridge and Patch 3 actually adds the config option to the schema and adds related unit tests. >[PATCH v3 4/5] qemu: command: add support for qemu options that Patch 4 adds the backend qemu commandline support for the options. It also adds relevant unit tests for the same. >[PATCH v3 5/5] NEWS: add new acpi pci hotplug options in the release Patch 5 adds the release notes for the next libvirt release. Changelog: v1: initial implementation. Had some bugs and missed some unit tests. v2: fixed bugs and added additional missing unit tests. v3: reorganized the patches as per Laine's suggestion. Added more details in commit messages. Added conf description in formatdomain.rst. Added changelog for next release. Notes: [1] One concrete example of why one might still want to use native hotplug with pcie-root-port controller is the fact that we are still discovering issues with acpi hotplug on PCIE. One such issue is: https://lists.gnu.org/archive/html/qemu-devel/2021-09/msg02146.html Another reason is that users have been using native hotplug on pcie root ports up until now. They have built and tested their systems based on native hotplug. They may not want to suddenly move to acpi based hotplug just because it is now the default in qemu. Supporting the option to chose one or the other through libvirt makes things simpler for end users. [2] The use case scenario described by Laine in https://listman.redhat.com/archives/libvir-list/2020-February/msg00110.html intentionally does not discuss i440fx and focusses solely on q35. I do realize that redhat has moved on from i440fx and currently efforts for new features are concentrated on q35 machines only. We have had some hard debates on this on the qemu mailing list before. The fact of the matter is that i440fx is not at 1-1 parity with q35. There are many users who are currenly using i440fx and are simply not ready to move to q35 without sacrificing some existing features they support today. For example https://wiki.qemu.org/images/4/4e/Q35.pdf lists some of q35 limitations. https://www.linux-kvm.org/images/0/06/2012-forum-Q35.pdf provides more information on the differences. Hence we need to solve the issue Laine has described in the above email for i440fx without adding additional bridges. Further, in Daniel Berrange's words from : https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg03012.html "From the upstream POV, there's been no decision / agreement to phase out PIIX, this is purely a RHEL downstream decision & plan. If other distros / users have a different POV, and find the feature useful, we should accept the patch if it meets the normal QEMU patch requirements. " Also to be noted that I have already experimented this qemu commandline option using libvirt passthrough feature as has been documented in http://blog.vmsplice.net/2011/04/how-to-pass-qemu-command-line-options.html This was only meant to be a short term solution until libvirt started supporting this natively. Supporting this option through libvirt would simplify their use case as well as add capability validations and graceful failure scenarios in case qemu did not support the option. [3] Finally, I implemented support for ``acpi-root-hotplug`` option in Qemu. Since adding the support for this option, I have not run away :-) I am still around, fixing other issues in the same subsystem in qemu and also now I have added myself as a reviewer of patches in this area. I will also be trying to support/maintain this new xml conf option in libvirt to the extent I can in future with the help of other experienced maintainers. Obviously this is all freelance work at this moment and is highly dependent on available free time.