Re: [PATCH RESEND v2 0/4] re-introduce <acpi-hotplug-bridge>

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On Tue, Apr 12, 2022 at 12:41 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Tue, Apr 12, 2022 at 09:52:26AM +0530, Ani Sinha wrote:
> > On Tue, Apr 12, 2022 at 9:50 AM Ani Sinha <ani@xxxxxxxxxxx> wrote:
> > >
> > > On Tue, Mar 8, 2022 at 10:28 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Mar 08, 2022 at 10:15:49PM +0530, Ani Sinha wrote:
> > > > >
> > > > > Change log:
> > > > > v2: rebased the patchset. Laine's response is appended at the end.
> > > > >
> > > > > I am re-introducing the patchset for <acpi-hotplug-bridge> which got
> > > > > reverted here few months back:
> > > > >
> > > > > https://www.spinics.net/linux/fedora/libvir/msg224089.html
> > > > >
> > > > > The reason for the reversal was that there seemed to be some
> > > > > instability/issues around the use of the qemu commandline which this
> > > > > patchset tries to support. In particular, some guest operating systems
> > > > > did not like the way QEMU was trying to disable native hotplug on pcie
> > > > > root ports. Subsequently, in QEMU 6.2, we have changed our mechanism
> > > > > using which we disable native hotplug. As I understand, we do not have
> > > > > any reported issues so far in 6.2 around this area. QEMU will enter a
> > > > > soft feature freeze in the first week of march in prep for 7.0 release.
> > > >
> > > > Right. But unfortunately we did not yet really work on
> > > > a sane interface for this.
> > > >
> > > > The way I see it, at high level we thinkably need two flags
> > > > - disable ACPI hotplug
> > > > - enable native hotplug (maybe separately for pci and pcie?)
>
> I still think this is the case.
>
> > > pci does not have native hotplug. so this would be applicable only for
> > > q35. For i440fx we have two separate flags already to disable acpi
> > > hotplug, one for root bus and another for bridges.
> > >
> > > >
> > > > and with both enabled guests actually can switch between
> > > > the two.
> > > >
> > > > This will at least reflect the hardware, so has a chance to be
> > > > stable.
> > > >
> > > > The big question however would be what is the actual use-case.
> > > > Without that this begs the question of why do we bother at all.
> > >
> > > To me the main motivation is as I have described here:
> > > https://listman.redhat.com/archives/libvir-list/2021-October/msg00068.html
> > >
> > > 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
>
> This one was fixed, right?

yes

>
>
> > > 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.
> >
> > Essentially what I do not like is that we are imposing acpi hotplug on
> > q35 for the entire community without giving them a choice to revert
> > back to native hotplug though libvirt.
>
> The reason qemu did it is because it was expected it's more or less
> transparent. Barring bugs bug hey, there's always bugs with any change.

Right and it takes time to say confidently that we have ironed out
almost all the issues.

>
> > >
> > > > To allow hotplug of bridges? If it is really necessary for us then
> > > > we should think hard about questions that surround this:
> > > >
> > > > - how does one hotplug a pcie switch?
> > > > - any way to use e.g. dynamic ACPI to support hotplug of bridges?
> > > > - do we want to bite the bullet and create an option for management
> > > >   to fully control guest memory layout including all pci devices?
> > > >
> > > >
> > > >
> > > > > Libvirt is also entering a new release cycle phaze. Hence, I am
> > > > > introducing this patchset early enough in the release cycles so that if
> > > > > we do see any issues on the qemu side during the rc0, rc1 cycles and if
> > > > > reversal of this patchset is again required, it can be done in time
> > > > > before the next libvirt release end of March.
> > > > >
> > > > > All the patches in this series had been previously reviewed. Some
> > > > > subsequent fixes were made after my initial patches were pushed. I have
> > > > > squashed all those fixes and consolidated them into four patches. I have
> > > > > also updated the documentation to reflect the new changes from the QEMU
> > > > > side and rebased my changes fixing the tests in the process.
> > > > >
> > > > > What changed in QEMU post version 6.1 ?
> > > > > =========================================
> > > > >
> > > > > We have made basically two major changes in QEMU. First is this change:
> > > > >
> > > > > (1) commit 211afe5c69b597acf85fdd577eb497f5be1ffbd8
> > > > > Author: Julia Suvorova <jusual@xxxxxxxxxx>
> > > > > Date:   Fri Nov 12 06:08:56 2021 -0500
> > > > >
> > > > >     hw/i386/acpi-build: Deny control on PCIe Native Hot-plug in _OSC
> > > > >
> > > > >     There are two ways to enable ACPI PCI Hot-plug:
> > > > >
> > > > >             * Disable the Hot-plug Capable bit on PCIe slots.
> > > > >
> > > > >     This was the first approach which led to regression [1-2], as
> > > > >     I/O space for a port is allocated only when it is hot-pluggable,
> > > > >     which is determined by HPC bit.
> > > > >
> > > > >             * Leave the HPC bit on and disable PCIe Native Hot-plug in _OSC
> > > > >               method.
> > > > >
> > > > >     This removes the (future) ability of hot-plugging switches with PCIe
> > > > >     Native hotplug since ACPI PCI Hot-plug only works with cold-plugged
> > > > >     bridges. If the user wants to explicitely use this feature, they can
> > > > >     disable ACPI PCI Hot-plug with:
> > > > >             --global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=off
> > > > >
> > > > >     Change the bit in _OSC method so that the OS selects ACPI PCI Hot-plug
> > > > >     instead of PCIe Native.
> > > > >
> > > > >     [1] https://gitlab.com/qemu-project/qemu/-/issues/641
> > > > >     [2] https://bugzilla.redhat.com/show_bug.cgi?id=2006409
> > > > >
> > > > >     Signed-off-by: Julia Suvorova <jusual@xxxxxxxxxx>
> > > > >     Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
> > > > >     Message-Id: <20211112110857.3116853-5-imammedo@xxxxxxxxxx>
> > > > >     Reviewed-by: Ani Sinha <ani@xxxxxxxxxxx>
> > > > >     Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >     Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >
> > > > >
> > > > > The patch description says it all. Instead of masking out the HPC bit in
> > > > > pcie slots, we keep them turned on. Instead, we do not advertize native
> > > > > hotplug capability for PCIE using _OSC control method. See section
> > > > > 6.2.11 in ACPI spec 6.2. At the same time, we turn on ACPI hotplug for
> > > > > these slots so now the guest OS can select ACPI hotplug instead.
> > > > >
> > > > > The second change is introduction of a property with which we keep the
> > > > > existing behavior for pc-q35-6.1 machines. This means HPC bit is masked
> > > > > and ACPI hotplug is enabled by default for pcie root ports.
> > > > > The QEMU commit is:
> > > > >
> > > > > (2) commit c318bef76206c2ecb6016e8e68c4ac6ff9a4c8cb
> > > > > Author: Julia Suvorova <jusual@xxxxxxxxxx>
> > > > > Date:   Fri Nov 12 06:08:54 2021 -0500
> > > > >
> > > > >     hw/acpi/ich9: Add compat prop to keep HPC bit set for 6.1 machine type
> > > > >
> > > > >     To solve issues [1-2] the Hot Plug Capable bit in PCIe Slots will be
> > > > >     turned on, while the switch to ACPI Hot-plug will be done in the
> > > > >     DSDT table.
> > > > >
> > > > >     Introducing 'x-keep-native-hpc' property disables the HPC bit only
> > > > >     in 6.1 and as a result keeps the forced 'reserve-io' on
> > > > >     pcie-root-ports in 6.1 too.
> > > > >
> > > > >     [1] https://gitlab.com/qemu-project/qemu/-/issues/641
> > > > >     [2] https://bugzilla.redhat.com/show_bug.cgi?id=2006409
> > > > >
> > > > >     Signed-off-by: Julia Suvorova <jusual@xxxxxxxxxx>
> > > > >     Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
> > > > >     Message-Id: <20211112110857.3116853-3-imammedo@xxxxxxxxxx>
> > > > >     Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >     Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >
> > > > > Lastly, as a related side note, because from QEMU 6.2 onwards, we do not
> > > > > mask out HPC bit in PCIE, the work done by this patch is no longer
> > > > > needed:
> > > > >
> > > > > (3) commit e2a6290aab578b2170c1f5909fa556385dc0d820
> > > > > Author: Marcel Apfelbaum <marcel.apfelbaum@xxxxxxxxx>
> > > > > Date:   Mon Aug 2 12:00:57 2021 +0300
> > > > >
> > > > >     hw/pcie-root-port: Fix hotplug for PCI devices requiring IO
> > > > >
> > > > >     Q35 has now ACPI hotplug enabled by default for PCI(e) devices.
> > > > >     As opposed to native PCIe hotplug, guests like Fedora 34
> > > > >     will not assign IO range to pcie-root-ports not supporting
> > > > >     native hotplug, resulting into a regression.
> > > > >
> > > > >     Reproduce by:
> > > > >         qemu-bin -M q35 -device pcie-root-port,id=p1 -monitor stdio
> > > > >         device_add e1000,bus=p1
> > > > >     In the Guest OS the respective pcie-root-port will have the IO range
> > > > >     disabled.
> > > > >
> > > > >     Fix it by setting the "reserve-io" hint capability of the
> > > > >     pcie-root-ports so the firmware will allocate the IO range instead.
> > > > >
> > > > >     Acked-by: Igor Mammedov <imammedo@xxxxxxxxxx>
> > > > >     Signed-off-by: Marcel Apfelbaum <marcel@xxxxxxxxxx>
> > > > >     Message-Id: <20210802090057.1709775-1-marcel@xxxxxxxxxx>
> > > > >     Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >     Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > >
> > > > >
> > > > > This is what commit (2) alludes to. In pc-q35-6.1 machines we do need
> > > > > patch (3) since we mask out HPC bit from pcie ports.
> > > > >
> > > > >
> > > > > I know this is convoluted mess. In fairness I am trying all I can in my
> > > > > spare time to help from the QEMU side. I am determined to see this
> > > > > patchset through into libvirt.
> > > > >
> > > > > Thanks
> > > > >
> > > > > Laine's comments ...
> > > > >
> > > > > My memory isn't completely clear, but I think there was also the issue
> > > > > that the option claims to enable ACPI hotplug when set to on, but
> > > > > instead what it actually does (in the Q35 case at least) is to enable
> > > > > native PCI hotplug when set to off (without actually disabling ACPI
> > > > > hotplug) and disable native PCI hotplug when set to on, or something
> > > > > like that. This ends up leaving it up to the guest OS to decide which
> > > > > type of hotplug to use, meaning its decision could override what's in
> > > > > the libvirt config, thus confusing everyone. Again, I probably have the
> > > > > details mixed up, but it was something like this.
> > > > >
> > > > > I asked mst about this this morning, and he suggested something that
> > > > > you've already done - Cc'ing the series to qemu-devel and the relevant
> > > > > maintainers so we can have a discussion with all involved parties about
> > > > > their opinions on whether we really should expose this existing option
> > > > > in libvirt, or if we should instead have two new options that are more
> > > > > orthogonal about enabling/disabling the two types of hotplug, so that
> > > > > libvirt config can more accurately represent what is being presented to
> > > > > the guest rather than a "best guess" of what we think the guest is going
> > > > > to do with what is presented.
> > > > >
> > > > > (Michael did also say that, with the current flurry of bug reports for
> > > > > the QEMU rc's, this discusion may not happen until closer to release
> > > > > when the bug reports die down. I know this doesn't mesh with your desire
> > > > > to "push now to allow for testing" (which in general would be a good
> > > > > thing if we were certain that we wanted the option like this and were
> > > > > just expecting some minor bugs that could be fixed), but my opinion is
> > > > > that 1) it's possible for anyone interested to test the functionality
> > > > > using <qemu:commandline>, and 2) we should avoid turning libvirt git
> > > > > into a revolving door of experiments. The only practical difference
> > > > > between using <qemu:commandline> and having a dedicated option is that
> > > > > the use of <qemu:commandline> causes the domain to be tainted, and the
> > > > > XML is a bit more complicated. But since the people we're talking about
> > > > > here will already have built their own libvirt binaries, the tainted
> > > > > status of any guests is irrelevant and the extra complexity of using
> > > > > <qemu:commandline> is probably trivial to them :-).
> > > > >
> > > > >
> > > > > Ani Sinha (4):
> > > > >   qemu: capablities: detect acpi-pci-hotplug-with-bridge-support
> > > > >   conf: introduce support for acpi-bridge-hotplug feature
> > > > >   qemu: command: add support for acpi-bridge-hotplug feature
> > > > >   NEWS: document new acpi pci hotplug config option
> > > > >
> > > > >  NEWS.rst                                      |  8 ++
> > > > >  docs/formatdomain.rst                         | 32 +++++++
> > > > >  docs/schemas/domaincommon.rng                 | 15 ++++
> > > > >  src/conf/domain_conf.c                        | 89 ++++++++++++++++++-
> > > > >  src/conf/domain_conf.h                        |  9 ++
> > > > >  src/qemu/qemu_capabilities.c                  |  4 +
> > > > >  src/qemu/qemu_capabilities.h                  |  3 +
> > > > >  src/qemu/qemu_command.c                       | 19 ++++
> > > > >  src/qemu/qemu_validate.c                      | 42 +++++++++
> > > > >  .../caps_6.1.0.x86_64.xml                     |  1 +
> > > > >  .../caps_6.2.0.x86_64.xml                     |  1 +
> > > > >  .../caps_7.0.0.x86_64.xml                     |  1 +
> > > > >  ...-hotplug-bridge-disable.aarch64-latest.err |  1 +
> > > > >  .../aarch64-acpi-hotplug-bridge-disable.xml   | 13 +++
> > > > >  ...-hotplug-bridge-disable.x86_64-latest.args | 35 ++++++++
> > > > >  .../pc-i440fx-acpi-hotplug-bridge-disable.xml | 36 ++++++++
> > > > >  .../pc-i440fx-acpi-hotplug-bridge-enable.xml  | 36 ++++++++
> > > > >  ...pi-hotplug-bridge-disable.x86_64-6.0.0.err |  1 +
> > > > >  ...-hotplug-bridge-disable.x86_64-latest.args | 38 ++++++++
> > > > >  .../q35-acpi-hotplug-bridge-disable.xml       | 53 +++++++++++
> > > > >  .../q35-acpi-hotplug-bridge-enable.xml        | 53 +++++++++++
> > > > >  tests/qemuxml2argvtest.c                      |  7 ++
> > > > >  ...i-hotplug-bridge-disable.x86_64-latest.xml |  1 +
> > > > >  ...pi-hotplug-bridge-enable.x86_64-latest.xml |  1 +
> > > > >  ...i-hotplug-bridge-disable.x86_64-latest.xml |  1 +
> > > > >  ...pi-hotplug-bridge-enable.x86_64-latest.xml |  1 +
> > > > >  tests/qemuxml2xmltest.c                       |  4 +
> > > > >  27 files changed, 504 insertions(+), 1 deletion(-)
> > > > >  create mode 100644 tests/qemuxml2argvdata/aarch64-acpi-hotplug-bridge-disable.aarch64-latest.err
> > > > >  create mode 100644 tests/qemuxml2argvdata/aarch64-acpi-hotplug-bridge-disable.xml
> > > > >  create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-hotplug-bridge-disable.x86_64-latest.args
> > > > >  create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-hotplug-bridge-disable.xml
> > > > >  create mode 100644 tests/qemuxml2argvdata/pc-i440fx-acpi-hotplug-bridge-enable.xml
> > > > >  create mode 100644 tests/qemuxml2argvdata/q35-acpi-hotplug-bridge-disable.x86_64-6.0.0.err
> > > > >  create mode 100644 tests/qemuxml2argvdata/q35-acpi-hotplug-bridge-disable.x86_64-latest.args
> > > > >  create mode 100644 tests/qemuxml2argvdata/q35-acpi-hotplug-bridge-disable.xml
> > > > >  create mode 100644 tests/qemuxml2argvdata/q35-acpi-hotplug-bridge-enable.xml
> > > > >  create mode 120000 tests/qemuxml2xmloutdata/pc-i440fx-acpi-hotplug-bridge-disable.x86_64-latest.xml
> > > > >  create mode 120000 tests/qemuxml2xmloutdata/pc-i440fx-acpi-hotplug-bridge-enable.x86_64-latest.xml
> > > > >  create mode 120000 tests/qemuxml2xmloutdata/q35-acpi-hotplug-bridge-disable.x86_64-latest.xml
> > > > >  create mode 120000 tests/qemuxml2xmloutdata/q35-acpi-hotplug-bridge-enable.x86_64-latest.xml
> > > > >
> > > > > --
> > > > > 2.25.1
> > > >
>




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