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

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. :-\