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

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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.

Ok so are we going to do something about this? I am still very unclear
as to what would be a sane interface both for i440fx and q35 (pci and
pcie).

>
> 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?)
>
> 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 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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