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

[Ani Sinha]

DanPB,
Could you please reiterate the suggestion regarding the placement of the
pci root hotplug xml you made in the irc channel?

On Fri, Sep 24, 2021 at 02:16 Laine Stump <laine at redhat.com> wrote:

> On 9/11/21 11:26 PM, Ani Sinha wrote:
> > 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>
>
> (before I get into a more radical discussion about different options -
> since we aren't exactly duplicating the QEMU option name anyway, what if
> we made these names more consistent, e.g. "acpi-hotplug-bridge" and
> "acpi-hotplug-root"?)
>
> I've thought quite a bit about whether to put these attributes here, or
> somewhere else, and I'm still undecided.
>
> My initial reaction to this was "PM == Power Management, and power
> management is all about suspend mode support. Hotplug isn't power
> management." But then you look at the name of the QEMU option and PM is
> right there in the name, and I guess it's *kind of related* (effectively
> suspending/resuming a single device), so maybe I'm thinking too narrowly.
>
> So are there alternate places that might fit the purpose of these new
> options better, rather than directly mimicking the QEMU option placement
> (for better or worse)? A couple alternative possibilities:
>
> 1) ****
>
> One possibility would be to include these new flags within the existing
> <acpi> subelement of <features>, which is already used to control
> whether the guest exposes ACPI to the guest *at all* (via adding
> "-no-acpi" to the QEMU commandline when <acpi> is missing - NB: this
> feature flag is currently supported only on x86 and aarch64 QEMU
> platforms, and ignored for all other hypervisors).
>
> Possibly the new flags could be put in something like this:
>
> <features>
>    <acpi>
>      <hotplug-bridge enabled='no'/>
>      <hotplug-root enabled='yes'/>
>    </acpi>
>    ...
> </features>
>
> But:
>
> * currently there are no subelements to <acpi>. So this isn't "extending
> according to an existing pattern".
>
> * even though the <features> element uses presence of a subelement to
> indicate "enabled" and absence of the subelement to indicate "disabled".
> But in the case of these new acpi bridge options we would need to
> explicitly have the "enabled='yes/no'" rather than just using presence
> of the option to mean "enabled" and absence to mean "disabled" because
> the default for "root-hotplug" up until now has been *enabled*, and the
> default for hotplug-bridge is different depending on machinetype. We
> need to continue working properly (and identically) with old/existing
> XML, but if we didn't have an "enabled" attribute for these new flags,
> there would be no way to tell the difference between "not specified" and
> "disabled", and so no way to disable the feature for a QEMU where the
> default was "enabled". (Why does this matter? Because I don't like the
> inconsistency that would arise from some feature flags using absense to
> mean "disabled" and some using it to mean "use the default".)
>
> * Having something in <features> in the domain XML kind of implies that
> the associated capability flags should be represented in the <features>
> section of the domain capabilities. For example, <acpi/> is listed under
> <features> in the output of virsh capabilities, separately from the flag
> indicating presence of the -no-acpi option. I'm not sure if we would
> need to add something there for these options if we moved them into
> <features> (seems a bit redundant to me to have it in both places, but
> I'm sure there are $reasons).
>
>
> 2) *****
>
> Alternately, there is an <acpi> subelement of <os>, which is currently
> used to add a SLIC table (some sort of software license table, which I'd
> never heard of before) using QEMU's -acpitable commandline option. It is
> also used somehow by the Xen driver.
>
> <os>
>    <acpi>
>      <table type='slic'>/path/to/slic.dat</table>
>      <hotplug-bridge enabled='no'/>
>      <hotplug-root enabled='yes'/>
>    </acpi>
>    ...
> </os>
>
> My problem with adding these new PCI controller acpi options to os/acpi
> is simply that it's in the <os> subelement, which is claimed elsewhere
> to be intended for OS boot options, and is used for things like
> specifying the path to a kernel / initrd to boot from.
>
> 3) ****
>
> A third option, suggested somewhere by Ani, would be to make a
> completely new top-level element, called something like <acpiHotplug>
> that would have separate attributes for the two flags, e.g.:
>
>     <acpiHotplug bridge='yes' root='yes'/>
>
> I dislike new toplevel options because they just seem so adhoc, as if
> the XML namespace is a cluttered, disorganized room. That reminds me too
> much of my own workspace, which is just... depressing.
>
> ****
>
> Since I always seem to spend *way too much time* worrying about naming,
> only to have it come out wrong in the end anyway, I'm looking for some
> other opinions. Counting the version that is in Ani's patch currently as
> option "0", which option do you all think is the best? Or is it
> completely unimportant?
>
> > 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:
>
> The "cold plugged bridges" term here throws me for a loop - it implies
> that hotplugging bridges is something that's supported, and I think it
> still isn't. Of course this is just the cover letter, so it won't go
> into git anywhere, but I think it should be enough to say "enables ACPI
> hotplug into non-root bus PCI bridges/ports".
>
> >
> > (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>
>
> So I'm curious - if the QEMU commandline also included "-no-acpi" along
> with these, what would happen? Would it be silently ignored? Generate an
> error? Or does -no-acpi only control the suspend support, and acpi
> hotplug is still available?
>
> >
> > 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")
>
> Interesting. So how was hotplug handled before this? With SHPC? I know
> there must be *some* kind of hotplug support in older QEMU, because
> RHEL6 QEMU supported hotplug, and it was based on qemu 0.12 or something
> ancient like that...
>
> > 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.
>
> Yes, sigh. I recall someone saying something like "if we switch to ACPI
> hotplug then all these bugs just go away and everything works" or
> something like that. Reality never matches the ideal picture we put in
> our brains.
>
> At least ACPI hotplug is only the default on new machinetypes (doesn't
> help much for management platforms that always just use "q35" every time
> they start a guest). And it can also cause problems with distro-specific
> machinetypes in downstream distros when they are rebased:
> https://bugzilla.redhat.com/2006409
>
> > 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.
>
> To be fair, aside from "support for Win2000/WinXP", none of the items on
> the "limitations" page of that slide deck is something that's impossible
> to do with a Q35 machinetype; it's just that accomplishing some things
> may be more complicated. But I understand your point. Mainly I brought
> it up because I wanted to be sure that we're adding these to fulfill an
> actual need, rather than just adding bulk for the sake of completeness,
> or to satisfy curiosity.
>
> > 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.
> >
> >
>
> Since I don't follow qemu-devel closely, I didn't have prior knowledge
> of exactly what the options did, and it was unclear in the earlier
> versions of your patches that what <acpi-hotplug-bridge enabled='no'/>
> did was to disable ACPI hotplug for the entire guest (which on Q35 means
> that native PCIe hotplug will be found/used, and on 440fx means that
> hotplug won't be possible (unless SHPC hotplugged is enabled)). Your
> exaplanation and documentation in this spin of the patches makes that
> all clear though, so I'm beyond the "what does this do and do we need
> it?" stage to the "are there any problems with the code?" stage, and
> that's what I'll try to address in my review of the patches.
>
>
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