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Re: [PATCH] qmp: Stabilize preconfig

From: Markus Armbruster
Subject: Re: [PATCH] qmp: Stabilize preconfig
Date: Wed, 03 Nov 2021 09:02:49 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux)

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Mon, Nov 01, 2021 at 03:37:58PM +0100, Michal Prívozník wrote:
>> On 10/25/21 2:19 PM, Markus Armbruster wrote:
>> > Michal Privoznik <mprivozn@redhat.com> writes:
>> > 
>> >> The -preconfig option and exit-preconfig command are around for
>> >> quite some time now. However, they are still marked as unstable.
>> >> This is suboptimal because it may block some upper layer in
>> >> consuming it. In this specific case - Libvirt avoids using
>> >> experimental features.
>> >>
>> >> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>> > 
>> > If I remember correctly, the motivation for -preconfig was NUMA
>> > configuration via QMP.  More uses may have appeared since.
>> > 
>> > Back then, I questioned the need for yet another option and yet another
>> > state: why not -S?
>> > 
>> > The answer boiled down to
>> > 
>> > 0. Yes, having just one would be a simpler and cleaner interface, but
>> > 
>> > 1. the godawful mess QEMU startup has become makes -S unsuitable for
>> >    some things we want to do, so we need -preconfig,
>> > 
>> > 2. which is in turn unsuitable for other things we want to do, so we
>> >    still need -S".
>> > 
>> > 3. Cleaning up the mess to the point where "simpler and cleaner" becomes
>> >    viable again is not in the cards right now.
>> I see a difference between the two. -preconfig starts QEMU in such a way
>> that its configuration can still be changed (in my particular use case
>> vCPUs can be assigned to NUMA nodes), while -S does not allow that. If
>> we had one state for both, then some commands must be forbidden from
>> executing as soon as 'cont' is issued. Moreover, those commands would
>> need to do much more than they are doing now (e.g. regenerate ACPI table
>> after each run). Subsequently, validating configuration would need to be
>> postponed until the first 'cont' because with just one state QEMU can't
>> know when the last config command was issued.

Doesn't all this apply to x-exit-preconfig already?

* Some commands are only allowed before x-exit-preconfig,
  e.g. set-numa-node.

* The complete (pre-)configuration is only available at
  x-exit-preconfig.  In particular, ACPI tables can be fixed only then.

>> Having said all of that, I'm not sure if -preconfig is the way to go or
>> we want to go the other way. I don't have a strong opinion.
> It feels like the scenario here is really just a specialization of the
> more general problem we want to be able to solve. Namely, we want to be
> able to start a bare QEMU and configure it entirely on the fly. IOW, we
> are really targetting for -preconfig to be able to do /all/ configuration,
> and with a new ELF binary, at which point -preconfig wouldn't exist, it
> would be the implicit default.

Whether -preconfig is the default or an option doesn't matter for
discussing the state machine.

> Libvirt primarily uses -S because it needs to query various aspects of
> QEMU's config before CPUs start executing, while QEMU can still be
> considered trustworthy (as it hasn't executed untrusted guest code 
> yet). eg we query vCPU PIDs so that we can apply CPU pinning to them. 
> We query the CPU model features so we can reflect what exact CPU 
> features we got from KVM. There are various other examples.

Which of the queries you need work only between x-exit-preconfig and -S?

Which of them could be made to work before x-exit-preconfig?

> I don't see this as inherantly tied to the -S option from a conceptual
> POV. We do have some ordering constraints, but they are not as crude
> as -preconfig / -S.  For querying vCPU PIDs, we have a constraint that
> the accelerator and machine objects must have been created. For querying
> CPU models, we have a similar constraint.  IOW, libvirt should be fine
> with doing /everything/ in a hypothetical -preconfig state, provided
> that we know about the ordering constraints wrt object creation.


> The secondary reason we use -S is that sometimes the mgmt app does
> not actually want the guest CPUs to start running - they actively
> want it in a paused state initially and will manually start CPUs
> later. One reason is to enable them to open the serial console
> backend before CPUs start, to guarantee that no console output is
> lost in that small startup window.  This is really the original
> purpose of -S.  This doesn't imply a need for -S. I'd say that
> -preconfig should essentially imply -S by default. If you're
> already doing lots of things via QMP, being required to issue
> a 'cont' command is no hardship.

I wonder whether we really have to step through three states

         x-exit-preconfig  cont
    preconfig ---> pre run ---> run

and not two

    pre run ---> run

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