On Tue, Sep 28, 2021 at 05:31:43PM +0530, Ani Sinha wrote:
>
>
> 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. :-\

I thought without -bridge is better, because we might want to hang
more PCI hotplug options off it later. The docs can clarify the
semantics

Regards,
Daniel
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