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Re: [Qemu-devel] [libvirt] [PULL 25/26] block: Remove deprecated -drive

From: Markus Armbruster
Subject: Re: [Qemu-devel] [libvirt] [PULL 25/26] block: Remove deprecated -drive option serial
Date: Thu, 12 Jul 2018 13:33:37 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Peter Krempa <address@hidden> writes:

> On Thu, Jul 12, 2018 at 08:59:44 +0200, Markus Armbruster wrote:
>> Peter Krempa <address@hidden> writes:
>> > On Tue, Jul 10, 2018 at 17:01:22 +0200, Cornelia Huck wrote:
>> >> On Tue, 10 Jul 2018 16:39:31 +0200
>> >> Peter Krempa <address@hidden> wrote:
> [...]
>> > An option is to do a automatic testing where one of this approaches will
>> > be enabled. For that you need a way to generate configurations which
>> > libvirt would use in real life. We have a rather big collection of XMLs
>> > which describe a valid configuration but the problem with using them on
>> > a real qemu is that most of the disk paths/network targets/other
>> > resources are made up and making them work with a real qemu would range
>> > from being painful to being impossible.
>> I sympathize.  However, it's not clear which one's harder, providing
>> environments for a sufficiently wide range of configurations (possibly
>> mockups), or hacking QEMU to do nothing but check configuration.  QEMU
> That's the main reason I think we should make it possible to use the
> data for the 'qemuxml2argv' test suite in libvirt. We require that new
> features are covered by this so that means that the testsuite is
> possibly the most comprehensive collection of libvirt configurations I
> know of.
> It's not perfect as we in many cases don't test any possible
> value but just try to excercise the code to generate them and others are
> left behind.
> Another historical problem was that we've defined a set of capabilities
> rather than using any real example to do this so many of the
> commandlines generated and tested are basically impossible to get in
> real life.
> That's why I added testing with real capabilities. We'll just need to
> generate a bunch of files to achieve full coverage here.
>> isn't designed for that, and configuration checking is intertwined with
>> everything else.  Complete disentanglement looks impractical to me.  I
> I agree. Getting anything special than the real codepath may create
> bubbles of problems still. On the other hand we'll need some guidance on
> what's sufficient to do to execute the deprecation detection code.
> This may require some coding style guidelines in qemu. E.g. no
> deprecation warnings after the vCPUs are started. Running a full
> operating system to check the warinigns would be utterly impractical.

Hot-plugging may get you deprecation warnings after vCPU start.  But I
get what you mean.  Rule of thumb: first check configuration is
well-formed, then do stuff that may fail when configuration asks for the
impossible, and only then do stuff that doesn't use configuration.

> Preferrably we would get away with starting qemu and waiting for the
> monitor to start.

We'll see how far that gets us.

>> guess we could do something useful at the QAPI level, though.  Yet
>> another reason to qapify the command line...
> That would be great, but I think that there's a subset of things that
> can be deprecated but can't be expressed by schema. In such case we
> still need to run the programatic checks to see.

There will always be stuff the schema can't express without complicating
the schema language a lot, and stuff the schema could express, but only
at a cost in readability we prefer not to pay.

To get the most mileage out of schema introspection, we should strive
for making things visible in there whenever practical.

>> > If we start from scratch you then lack coverage.
>> >
>> >> If we fail with exit(1), can libvirt check any message that is logged
>> >> right before that?
>> >
>> > Yes we currently use this for very early failures which occur prior to
>> > the monitor working.
>> >
>> >> > To make any reasonable use of -no-deprecated-options we'd also need
>> >> > something that simulates qemu startup (no resources are touched in fact)
>> >> > so that we can run it against the testsuite. Otherwise the use will be
>> >> > limited to developers using it with the configuration they are
>> >> > currently testing.
>> >>> 
>> >> Would that moan loudly that you should poke the libvirt developers if
>> >> some kind of testsuite failure is detected? Or am I misunderstanding?
>> >
>> > Generally it should make somebody complain. But there is a problem.
>> > Since we are talking deprecation it can't be enabled by default. And by
>> > not making it default most of the users will not enable that option.
>> I don't think end users should do the work of catching use of deprecated
>> features.  It's a CI job.
>> In a CI context, we don't need fancy QMP infrastructure to communicate
>> "you used a deprecated feature", we can get away with printing an
>> explanation to stderr and exit(1).  That should make CI fail, and the
>> failure should make a developer read the explanation.  To unbreak CI, he
>> can either fix the problem right away, or file a BZ and suppress the CI
>> failure until it's fixed, say by downgrading --deprecated=error to
>> --deprecated=warn.
> Definitely. Plain untranslated error message is fine. The only thing is
> that it should be easy to detect. exit(1) is that solution. Or rather
> exit($VALUE_SPECIFIC_FOR_DEPRECATION) so that we can automatically
> discriminate test failures from deprecation warnings.

We'll have to search for of exit(X), where X is a bad idea.

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