[libvirt] [Qemu-devel] [PATCH 2/2] qapi: deprecate implicit filters

Christophe de Dinechin dinechin at redhat.com
Fri Nov 22 11:32:33 UTC 2019

> On 22 Nov 2019, at 09:41, Markus Armbruster <armbru at redhat.com> wrote:
> Reviving this old thread, because I'd like to connect it to more recent
> discussions.
> Christophe de Dinechin <dinechin at redhat.com <mailto:dinechin at redhat.com>> writes:
>> Markus Armbruster writes:
>>> Peter Krempa <pkrempa at redhat.com> writes:
>> [...]
>>>> From my experience users report non-fatal messages mostly only if it is
>>>> spamming the system log. One of instances are very unlikely to be
>>>> noticed.
>>>> In my experience it's better to notify us in libvirt of such change and
>>>> we will try our best to fix it.
>>> How to best alert the layers above QEMU was one of the topic of the KVM
>>> Forum 2018 BoF on deprecating stuff.  Minutes:
>>>    Message-ID: <87mur0ls8o.fsf at dusky.pond.sub.org>
>>>    https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg05828.html
>>> Relevant part:
>>> * We need to communicate "you're using something that is deprecated".
>>>  How?  Right now, we print a deprecation message.  Okay when humans use
>>>  QEMU directly in a shell.  However, when QEMU sits at the bottom of a
>>>  software stack, the message will likely end up in a log file that is
>>>  effectively write-only.
>>>  - The one way to get people read log files is crashing their
>>>    application.  A command line option --future could make QEMU crash
>>>    right after printing a deprecation message.  This could help with
>>>    finding use of deprecated features in a testing environment.
>>>  - A less destructive way to grab people's attention is to make things
>>>    run really, really slow: have QEMU go to sleep for a while after
>>>    printing a deprecation message.
>>>  - We can also pass the buck to the next layer up: emit a QMP event.
>>>    Sadly, by the time the next layer connects to QMP, plenty of stuff
>>>    already happened.  We'd have to buffer deprecation events somehow.
>>>    What would libvirt do with such an event?  Log it, taint the domain,
>>>    emit a (libvirt) event to pass it on to the next layer up.
>>>  - A completely different idea is to have a configuratin linter.  To
>>>    support doing this at the libvirt level, QEMU could expose "is
>>>    deprecated" in interface introspection.  Feels feasible for QMP,
>>>    where we already have sufficiently expressive introspection.  For
>>>    CLI, we'd first have to provide that (but we want that anyway).
>>>  - We might also want to dispay deprecation messages in QEMU's GUI
>>>    somehow, or on serial consoles.
>> Sorry for catching up late, this mail thread happened during my PTO.
>> I remember bringing up at the time [1] that the correct solution needs
>> to take into account usage models that vary from
>> - a workstation case, where displaying an error box is easy and
>>  convenient,
>> - to local headless VMs where system-level notification would do the job
>>  better, allowing us to leverage things like system-wide email notifications
>> - to large-scale collections of VMs managed by some layered product,
>>  where the correct reporting would be through something like Insights,
>>  i.e. you don't scan individual logs, you want something like "913 VMs
>>  are using deprecated X"
>> To me, that implies that we need to have a clear division of roles, with
>> a standard way to
>> a) produce the errors,
>> b) propagate them,
>> c) consume them (at least up to libvirt)
>> Notice that this work has already been done for "real" errors,
>> i.e. there is a real QAPI notion of "errors". AFAICT, warn_report does
>> not connect to it, though, it goes through error_vprintf which is really
>> just basic logging.
>> So would it make sense to:
>> 1. Add a deprecation_report() alongside warn_report()?
> "Grepability" alone would make that worthwhile, I think.
>> 2. Connect warn_report() and all the error_vprintf output to QAPI,
>>   e.g. using John's suggestion of adding the messages using some
>>   "warning" or "deprecated" tag?
> This is the difficult part.  See my "Exposing feature deprecation to
> machine clients (was: [Qemu-devel] [PATCH 2/2] qapi: deprecate implicit
> filters)" in this thread.  Quote:
>    Propagating errors is painful.  It has caused massive churn all over the
>    place.
>    I don't think we can hitch deprecation info to the existing error
>    propagation, since we need to take the success path back to the QMP
>    core, not an error path.
>    Propagating a second object for warnings... thanks, but no thanks.
>    The QMP core could provide a function for recording deprecation info for
>    the currently executing QMP command.  This is how we used to record
>    errors in QMP commands, until Anthony rammed through what we have now.
>    The commit messages (e.g. d5ec4f27c38) provide no justification.  I
>    dimly recall adamant (oral?) claims that recording errors in the Monitor
>    object cannot work for us.
>    I smell a swamp.

This looks swampy the way you describe it, but this make me think that
the recorder library may provide a somewhat elegant solution to this.

First, my presupposition here is that part of the issues you are describing
is because there is no “one size fits all” reporting of errors, warnings, etc.
Essentially, it’s a policy that is defined outside of libvirt or qemu, and not
all consumers will want the same policy.

The way the recorder library addresses that is to have as many recorders
as you need, and allowing the behavior for each recorder to be configured
individually at run-time, e.g. through env variables.

If you couple that with a naming convention for the recorders, you can
set the policy externally in a rather flexible way. For example, I will typically
have recorders that end with _error or _warning or _info or _verbose.
We could have a _deprecated convention just the same.

With a simple syntax, you can state that you want tracing on
all warnings and errors by activating a regexp like “.*_(warning|error)”,
or disable anything related to deprecation with “.*_deprecated.*=0”.
It wouldn’t be much of a stretch to have similar policies for “create a
qapi error object” or even “abort the process”.

This approach means that we would get to define the default policy
for the various deprecation cases, but that the user would ultimately
be able to override, maybe even individually, e.g.:

	// I don’t care about deprecation at all

	// Log all deprecation messages, do nothing else with it

	// Log most deprecation, but ignore “blink_tag” deprecation
	// messages and abort on “serial_uart” deprecation message

>    Can we avoid plumbing deprecation info from command code to QMP core?
>    Only if the QMP core itself can recognize deprecated interfaces.  I
>    believe it can for the cases we can expose in introspecion.  Let me
>    explain.

The recorder above (as it exists today) only deals with logging and reporting
messages. It provides no help whatsoever for introspection, which I assume
you would use to warn “ahead of time” that something is deprecated, right?

>    Kevin recently added "features" to the QAPI schema language.  The
>    implementation is incomplete, but that's detail.  The idea is to tack a
>    "deprecated" feature to deprecated stuff in the QAPI schema.
>    Commands and arguments need to support features for that.
>    Implementation should be relatively straightforward.
>    Deprecating an argument's optionalness may require a
>    "optional-deprecated" feature.  I've seen more elegant designs, but I've
>    also seen plenty of uglier ones.
>    Note that features are tied to schema syntax.  To express semantically
>    conditional deprecation like "if you specify argument FOO, then not
>    specifying argument BAR is deprecated", we'd have to add a language for
>    these conditions.  Uh, not now, maybe never.

For these cases, we can still fallback to C code, no? As long as the
“report_deprecated()” being used is somewhat consistent between
the two, the fact that there is a “short, common case” for optiions
and a “complicated, less common case” would be invisible to users.

>    The primary use of having deprecation defined in the QAPI schema is
>    introspection.  The BoF minutes mention this, too.
>    A secondary use could be detecting use of deprecated features right in
>    the QMP core.  No need for ad hoc code in commands, no need for plumbing
>    information from there to the QMP core.
>    I'd like to pursue this idea, then see how well it suits our deprecation
>    needs.
> I've since explored this idea in
>    [RFC PATCH 00/19] Configurable policy for handling deprecated interfaces
>    Message-Id: <20191024123458.13505-1-armbru at redhat.com <mailto:20191024123458.13505-1-armbru at redhat.com>>
>    https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg06534.html <https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg06534.html>    

I had not paid attention to this one. I’ll take a look and get back to you.

>> 3. Teach libvirt how to consume that new tag and pass it along?
> Pertinent:
> Message-ID: <20191024140146.GC8381 at redhat.com <mailto:20191024140146.GC8381 at redhat.com>>
> https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg06604.html <https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg06604.html>
> Quoting Dan's conclusion:
>    In summary, it is probably reasonable to include this info in the QMP
>    command reply, but don't expect much to be done with it beyond possibly
>    writing it to a log file.
>> [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg06131.html <https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg06131.html>


>> --
>> Cheers,
>> Christophe de Dinechin (IRC c3d)

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