[PATCH v7 0/4] Add support to enable/disable hotplug on pci-root controller

Fri Oct 1 21:25:04 UTC 2021

On 10/1/21 5:29 AM, Ani Sinha wrote:
> changelog:
> 
> v7: rebased the patches post libvirt release 7.8. Adjusted NEWS.rst to reflect 7.9 (unreleased)
>      version. Changed version info for new config option in formatdomain.rst as well.
>      Added reviewed-by tags. Also in formatdomain.rst added information on what type of hotplug
>      (ACPI/native etc) are affected by the setting.
> v6: incorporated Jan's suggestions.
> v5: incorporated suggestions from Laine for patches #2 and #3. The qemu command line
>      is now added in a new function and called from qemuBuildControllersByTypeCommandLine().
>      Output xmls are now symlinked to input xmls for unit tests.
> v4: split the original patchset into a pci-root controller specific patch series.
>      also the libvirt conf is now a sub-element of the pci-root controller as was
>      suggested by Dan Berrange. Please see discussion here:
>      https://listman.redhat.com/archives/libvir-list/2021-September/msg00839.html
> 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.
> v2: fixed bugs and added additional missing unit tests.
> v1: initial implementation. Had some bugs and missed some unit tests
> 
> This patchset introduces libvirt xml support to enable/disable hotplug on the
> pci-root controller. It adds a 'target' subelement for the pci-root controller
> with a 'hotplug' property. This property can be used to enable or disable
> hotplug for the pci-root controller. For example, in order to disable hotplug
> on the pci-root controller, one has to use set '<target hotplug='off'>' as
> shown below:
> 
> <controller type='pci' model='pci-root'>
>    <target hotplug='off'/>
> </controller>
> 
> '<target hotplug='on'>' option would enable hotplug for pci-root controller.
> This is also the default value. This option is only available for pc machine
> types and is applicable for qemu only/kvm accelerator onlt.This feature was
> introduced from qemu version 5.2 with the following change in qemu repository:
> 
> 3d7e78aa7777f ("Introduce a new flag for i440fx to disable PCI hotplug on the root bus")
> 
> The above qemu commit describes some reasons why users might to disable hotplug
> on PCI root buses [1].
> 
> The corresponding commandline option to qemu for x86 guests is:
> 
> -global PIIX4_PM.acpi-root-pci-hotplug=<off/on>
> 
> Notes:
> 
> 1. 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.
> 
> 
> Ani Sinha (4):
>    qemu: capablities: detect presence of acpi-root-pci-hotplug for i440fx
>      machines
>    conf: introduce option to enable/disable pci hotplug on pci-root
>      controller
>    qemu: command: add support to enable/disable hotplug on pci-root
>      controller
>    NEWS: release note the new hotplug enable/disable option on pci-root
>      controller
> 
>   NEWS.rst                                      |  6 ++++
>   docs/formatdomain.rst                         | 14 ++++++---
>   src/qemu/qemu_capabilities.c                  |  4 +++
>   src/qemu/qemu_capabilities.h                  |  3 ++
>   src/qemu/qemu_command.c                       | 17 ++++++++++
>   src/qemu/qemu_validate.c                      |  9 +++++-
>   .../caps_5.2.0.x86_64.xml                     |  1 +
>   .../caps_6.0.0.x86_64.xml                     |  1 +
>   .../caps_6.1.0.x86_64.xml                     |  1 +
>   .../pc-i440fx-acpi-root-hotplug-disable.args  | 31 +++++++++++++++++++
>   .../pc-i440fx-acpi-root-hotplug-disable.err   |  1 +
>   .../pc-i440fx-acpi-root-hotplug-disable.xml   | 30 ++++++++++++++++++
>   .../pc-i440fx-acpi-root-hotplug-enable.xml    | 30 ++++++++++++++++++
>   tests/qemuxml2argvtest.c                      |  3 ++
>   .../pc-i440fx-acpi-root-hotplug-disable.xml   |  1 +
>   .../pc-i440fx-acpi-root-hotplug-enable.xml    |  1 +
>   tests/qemuxml2xmltest.c                       |  4 +++
>   17 files changed, 151 insertions(+), 6 deletions(-)
>   create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-root-hotplug-disable.args
>   create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-root-hotplug-disable.err
>   create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-root-hotplug-disable.xml
>   create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-root-hotplug-enable.xml
>   create mode 120000 tests/qemuxml2xmloutdata/pc-i440fx-acpi-root-hotplug-disable.xml
>   create mode 120000 tests/qemuxml2xmloutdata/pc-i440fx-acpi-root-hotplug-enable.xml
> 

Okay, I made slight changes to the wording in a couple of the commit 
messages and documentation to emphasize that it only applies to i440fx 
machinetypes, and pushed the series.

Thanks for the contribution!