[libvirt] [Qemu-devel] [PATCH v7 1/3] qmp: adding 'wakeup-suspend-support' in query-target

[Markus Armbruster]

Eduardo Habkost <ehabkost at redhat.com> writes:

> On Wed, May 23, 2018 at 11:17:55AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost at redhat.com> writes:
>> > On Mon, May 21, 2018 at 04:46:36PM -0300, Daniel Henrique Barboza wrote:
> [...]
>> >> Since no objection was made back then, this logic was put into query-target
>> >> starting
>> >> in v2. Still, I don't have any favorites though: query-target looks ok,
>> >> query-machine
>> >> looks ok and a new API looks ok too. It's all about what makes (more) sense
>> >> in the
>> >> management level, I think.
>> >
>> > I understand the original objection from Eric: having to add a
>> > new command for every runtime flag we want to expose to the user
>> > looks wrong to me.
>> 
>> Agreed.
>> 
>> > However, extending query-machines and query-target looks wrong
>> > too, however.  query-target looks wrong because this not a
>> > property of the target.  query-machines is wrong because this is
>> > not a static property of the machine-type, but of the running
>> > machine instance.
>> 
>> Of the two, query-machines looks less wrong.
>> 
>> Arguably, -no-acpi should not exist.  It's an ad hoc flag that sneakily
>> splits a few machine types into two variants, with and without ACPI.
>> It's silently ignored for other machine types, even APCI-capable ones.
>> 
>> If the machine type variants with and without ACPI were separate types,
>> wakeup-suspend-support would be a static property of the machine type.
>> 
>> However, "separate types" probably doesn't scale: I'm afraid we'd end up
>> with an undesirable number of machine types.  Avoiding that is exactly
>> why we have machine types with configurable options.  I suspect that's
>> how ACPI should be configured (if at all).
>> 
>> So, should we make -no-acpi sugar for a machine type parameter?  And
>> then deprecate -no-acpi for good measure?
>
> I think we should.

Would you like to take care of it?

>> > Can we have a new query command that could be an obvious
>> > container for simple machine capabilities that are not static?  A
>> > name like "query-machine" would be generic enough for that, I
>> > guess.
>> 
>> Having command names differ only in a single letter is awkward, but
>> let's focus on things other than naming now, and use
>> query-current-machine like a working title.
>> 
>> query-machines is wrong because wakeup-suspend-support isn't static for
>> some machine types.
>> 
>> query-current-machine is also kind of wrong because
>> wakeup-suspend-support *is* static for most machine types.
>
> The most appropriate solution depends a lot on how/when
> management software needs to query this.

Right.

> If they only need to query it at runtime for a running VM,
> there's no reason for us to go of our way and add complexity just
> to make it look like static data in query-machines.
>
> On the other hand, if they really need to query it before
> configuring/starting a VM, it won't be useful at all to make it
> available only at runtime.
>
> Daniel, when/how exactly software would need to query the new
> flag?
>
>
>> Worse, a machine type property that is static for all machine types now
>> could conceivably become dynamic when we add a machine type
>> configuration knob.
>> 
>
> This isn't the first time a machine capability that seems static
> actually depends on other configuration arguments.  We will
> probably need to address this eventually.

Then the best time to address it is now, provided we can :)

>> Would a way to tie the property to the configuration knob help?
>> Something like wakeup-suspend-support taking values true (supported),
>> false (not supported), and "acpi" (supported if machine type
>> configuration knob "acpi" is switched on).
>> 
>
> I would prefer a more generic mechanism.  Maybe make
> 'query-machines' accept a 'machine-options' argument?

This can support arbitrary configuration dependencies, unlike my
proposal.  However, I fear combinatorial explosion would make querying
anything but "default configuration" and "current configuration"
impractical, and "default configuration" would be basically useless, as
you can't predict how arguments will affect the value query-machines.

Whether this is an issue depends on how management software wants to use
query-machines.

Whether the ability to support arbitrary configuration dependencies is a
useful feature or an invitation to stupid stunts is another open
question :)

Here's a synthesis of the two proposals: have query-machines spell out
which of its results are determinate, and which configuration bits need
to be supplied to resolve the indeterminate ones.  For machine type
"pc-q35-*", wakeup-suspend-support would always yield true, but for
"pc-i440fx-*" it would return true when passed an acpi: true argument,
false when passed an acpi: false argument, and an encoding of
"indeterminate, you need to pass an acpi argument to learn more" when
passed no acpi argument.

I'm not saying this synthesis makes sense, I'm just exploring the design
space.

We need input from libvirt guys.