[libvirt] [PATCH RESEND v2 0/4] re-introduce <acpi-hotplug-bridge>
Ani Sinha
ani at anisinha.ca
Tue Apr 12 04:22:26 UTC 2022
On Tue, Apr 12, 2022 at 9:50 AM Ani Sinha <ani at anisinha.ca> wrote:
>
> On Tue, Mar 8, 2022 at 10:28 PM Michael S. Tsirkin <mst at redhat.com> 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?)
>
> 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
> 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.
>
> > 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 at redhat.com>
> > > 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 at redhat.com>
> > > Signed-off-by: Igor Mammedov <imammedo at redhat.com>
> > > Message-Id: <20211112110857.3116853-5-imammedo at redhat.com>
> > > Reviewed-by: Ani Sinha <ani at anisinha.ca>
> > > Reviewed-by: Michael S. Tsirkin <mst at redhat.com>
> > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > >
> > >
> > > 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 at redhat.com>
> > > 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 at redhat.com>
> > > Signed-off-by: Igor Mammedov <imammedo at redhat.com>
> > > Message-Id: <20211112110857.3116853-3-imammedo at redhat.com>
> > > Reviewed-by: Michael S. Tsirkin <mst at redhat.com>
> > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > >
> > > 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 at gmail.com>
> > > 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 at redhat.com>
> > > Signed-off-by: Marcel Apfelbaum <marcel at redhat.com>
> > > Message-Id: <20210802090057.1709775-1-marcel at redhat.com>
> > > Reviewed-by: Michael S. Tsirkin <mst at redhat.com>
> > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > >
> > >
> > > 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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