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

[Ani Sinha]

On Tue, 28 Sep 2021, Daniel P. Berrangé wrote:

> On Thu, Sep 23, 2021 at 04:46:38PM -0400, Laine Stump 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.
>
> I had the same reaction.  Even if QEMU hangs it off a "_PM" device,
> I feel it is a pretty wierd location from libvirt POV to put this.
>
> > 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).
>
> Essentially <features> has become a dumping ground for adhoc global
> properties. So in that sense it probably is the best fit for this.
>
> If we don't want to touch th existing <acpi> element for fear of
> back compat issues, we could have
>
>    <pci-hotplug acpi="yes|no"/>
>
> for the acpi-pci-hotplug-with-bridge-support   setting ?
>

Since this is pci bridge related setting, maybe we should have:

<pci-hotplug-bridge acpi="yes|no"/>

Although in that case, the user should be aware that pcie-root-ports are
like bridges. But if we do not have -bridge, then it does not convey the
fact that this setting does not apply to pci-root bus on i440fx. :-\

>
> > 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.
>
> Yeah, we've kind of abused <os> a little with adding <acpi> under
> that. I can see why we did it, as its another blob kinda like the
> loader blob, but it was probabl a mistake.
>
> >
> > 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.
>
> Agreed, lets not add more top level pieces.
>
> Regards,
> Daniel
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