Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
> On 22 Nov 2019, at 09:41, Markus Armbruster wrote: > > Reviving this old thread, because I'd like to connect it to more recent > discussions. > > Christophe de Dinechin mailto:dinec...@redhat.com>> > writes: > >> Markus Armbruster writes: >> >>> Peter Krempa 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@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
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
Reviving this old thread, because I'd like to connect it to more recent discussions. Christophe de Dinechin writes: > Markus Armbruster writes: > >> Peter Krempa 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@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. 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.
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
Am 30.08.2019 um 20:11 hat John Snow geschrieben:
>
>
> On 8/30/19 6:07 AM, Christophe de Dinechin wrote:
> > Without having looked at the code much, I think I would
> >
> > 1. extend the existing QAPI error to support warnings, deprecations and
> >info messages. The first problem I see is that there is no error, so
> >we may sometimes need to create one when there was none before. And
> >of course make sure that this does not ultimately show as an error,
> >but as a success with additional annotations.
> >
>
> I assume this might be a chance to consolidate all of the methodologies
> we use for actually checking if there was an error or not. There have
> been many and I am sure Markus can give us a history lesson if it's
> warranted.
>
> Generally, there's a few paradigms I see a lot:
>
> 1. Rely on an error return code being produced by the called function.
> The caller trusts that errp was set. This is one of my favorite methods,
> because it has the least scaffolding.
This one is convenient to use, but the problem is that nobody enforces
that errp is always set if ret < 0, and that it's not set for ret == 0.
So in a way it is error-prone because it allows inconsistencies.
> 2. Pass errp directly to the called function, and check for null after
> return. I don't like this method very much, because of confusion with:
I mainly don't like this very much because it's simply wrong.
Callers can pass errp = NULL if they aren't interested in error
information. If you directly pass errp, you can't check *errp because
errp could be NULL.
So directly passing errp makes the code simpler, but only use it in
functions where you don't intend to check whether an error is set.
> 3. Create a local error object; check THAT for null, and propagate the
> error to the common error object. I think Markus has explained why we
> have this code 50 times, and I forget again minutes later.
local_err exists for those cases where you need to check the error
object before passing it to the caller. (And obviously for those cases
where you don't want to pass it to the caller, but do something like
error_report_err().)
> If we want to expand the concept of the error object into something that
> encompasses hints, warnings and deprecations*, checking for null is no
> longer appropriate. It might be a good chance to make our error
> propagation story more consistent, too.
>
> We could unify with a helper like this, I think, if I'm not forgetting
> some crucial usage detail:
>
> subroutine(foo, bar, errp);
> if (failure(errp)) {
> error_append_hint(errp, "Lorem ipsum, ...");
> cleanup();
> return;
> }
>
> We would then always use this pattern that operates directly on the
> caller's errp instead of creating local error objects to allow hints and
> warnings to accumulate.
There are two parts to the change that you imply:
1. Forbid passing errp = NULL to any function so that *errp can always
be checked. This gets rid of local_err in the intermediate function,
but may require the introduction of new local_err variables in
top-level callers which ignore the error information.
2. Introduce failure(errp) to replace errp != NULL because we want Error
to contain warnings and notes, too. Currently, it can contain only
exactly one error, so this would be a major change.
Note that the change wouldn't make the existing 'if (errp)' checks
build failures, so getting confident that we found and replaced all
of them is going to be hard.
Essentially, you'd probably want to replace Error with a new type so
that the compiler will at least be able to tell which places have been
converted and which haven't.
And then, you'd have to touch every single function that does something
with errors. This is a huge change across the whole source tree.
I doubt it's worth the effort.
> > Second, why not report the use of deprecated features? I don't fully buy
> > the rationale that libvirt engages the features, because it does not do
> > it on its own, it does it because the user made some specific request.
>
> Because the user didn't request those specific QMP features, they asked
> for the VM to start, or to stop, or they asked for a backup, or a snapshot.
>
> On my desktop, I am not really too interested in knowing if XFCE is
> using deprecated features of xorg or wayland. I didn't tell it to use
> them and I have no real power or control over that. It's nice if I'm a
> developer, but as a user, it's noise.
>
> So a development log seems right for these, but not user-visible
> interruptions.
I agree, it's not the high-level operation the user requested that is
deprecated, but just the specific implementation libvirt uses to perform
the operation on a QEMU VM.
The expected response to a deprecation notice is that a libvirt update
makes it go away by using more modern interfaces, not that the user
changes their workflow.
Kevin
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
On 8/30/19 6:07 AM, Christophe de Dinechin wrote:
>
> John Snow writes:
>
>> On 8/29/19 12:45 PM, Christophe de Dinechin wrote:
>>>
> [...]
>
>>> 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,
>>
>> I started replying to this thread to the other mail you sent; I think
>> this is going to be fairly involved. I wouldn't mind being proven wrong
>> though.
>
> Yes, I think it does look involved, but mostly for historical reasons.
> In other words, what is complicated is preserving the historical
> behaviors so as to not break existing consumers.
>
>>
>>> 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()?
>>>
>>
>> Where's that get routed to? just an error_vprintf style situation?
>
> Yes, but see below.
>
>>
>>> 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?
>>>
>>
>> How do you correlate them?
>
> Without having looked at the code much, I think I would
>
> 1. extend the existing QAPI error to support warnings, deprecations and
>info messages. The first problem I see is that there is no error, so
>we may sometimes need to create one when there was none before. And
>of course make sure that this does not ultimately show as an error,
>but as a success with additional annotations.
>
I assume this might be a chance to consolidate all of the methodologies
we use for actually checking if there was an error or not. There have
been many and I am sure Markus can give us a history lesson if it's
warranted.
Generally, there's a few paradigms I see a lot:
1. Rely on an error return code being produced by the called function.
The caller trusts that errp was set. This is one of my favorite methods,
because it has the least scaffolding.
2. Pass errp directly to the called function, and check for null after
return. I don't like this method very much, because of confusion with:
3. Create a local error object; check THAT for null, and propagate the
error to the common error object. I think Markus has explained why we
have this code 50 times, and I forget again minutes later.
If we want to expand the concept of the error object into something that
encompasses hints, warnings and deprecations*, checking for null is no
longer appropriate. It might be a good chance to make our error
propagation story more consistent, too.
We could unify with a helper like this, I think, if I'm not forgetting
some crucial usage detail:
subroutine(foo, bar, errp);
if (failure(errp)) {
error_append_hint(errp, "Lorem ipsum, ...");
cleanup();
return;
}
We would then always use this pattern that operates directly on the
caller's errp instead of creating local error objects to allow hints and
warnings to accumulate.
(* I'm not proposing all three in a concrete way, but just referencing
the general class of semantic non-error information we may want to
propagate, however we end up deciding to model it.)
> 2. replace the current "link + if" switching for error_vprintf with some
>actual notification mechanism, with one option routine to
>monitor_vprintf, one to stderr, one to log file, and then an
>additional path that would post a newly minted qapi warning.
>
>>
>>> 3. Teach libvirt how to consume that new tag and pass it along?
>>>
>>
>> I think it's not libvirt's job to pass it along, exactly -- libvirt made
>> the decision for which features to engage in QEMU, not the end user.
>
> First, by "pass along", I meant to possible layered products or
> management software. We don't necessarily need a new virErrorLevel,
> deprecation could be a warning with some special domain,
> e.g. VIR_FROM_DEPRECATION.
>
SHOULD it pass it along? Libvirt knows what QMP is invoking, that should
be opaque to upper layers. a "DEPRECATION"
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
John Snow writes: > On 8/29/19 12:45 PM, Christophe de Dinechin wrote: >> [...] >> 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, > > I started replying to this thread to the other mail you sent; I think > this is going to be fairly involved. I wouldn't mind being proven wrong > though. Yes, I think it does look involved, but mostly for historical reasons. In other words, what is complicated is preserving the historical behaviors so as to not break existing consumers. > >> 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()? >> > > Where's that get routed to? just an error_vprintf style situation? Yes, but see below. > >> 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? >> > > How do you correlate them? Without having looked at the code much, I think I would 1. extend the existing QAPI error to support warnings, deprecations and info messages. The first problem I see is that there is no error, so we may sometimes need to create one when there was none before. And of course make sure that this does not ultimately show as an error, but as a success with additional annotations. 2. replace the current "link + if" switching for error_vprintf with some actual notification mechanism, with one option routine to monitor_vprintf, one to stderr, one to log file, and then an additional path that would post a newly minted qapi warning. > >> 3. Teach libvirt how to consume that new tag and pass it along? >> > > I think it's not libvirt's job to pass it along, exactly -- libvirt made > the decision for which features to engage in QEMU, not the end user. First, by "pass along", I meant to possible layered products or management software. We don't necessarily need a new virErrorLevel, deprecation could be a warning with some special domain, e.g. VIR_FROM_DEPRECATION. There may be a need to add some API here. Looking at the code, it's not obvious to me that libvirt has any notion of error priority. In other words, if you raise an error then a warning, you get the warning as the last error, right? Second, why not report the use of deprecated features? I don't fully buy the rationale that libvirt engages the features, because it does not do it on its own, it does it because the user made some specific request. This point of view also seems to require that libvirt or the user should know ahead of time it's about to engage a deprecated feature. To me, the problem is precisely that neither libvirt nor the user knows, which is why we are discussing how to best make it known. > > If the user upgrades QEMU but not libvirt, it's not really anything they > have control over and they shouldn't be pestered with such things. > > However, if libvirt accidentally released a version that engages > deprecated behavior (and were unaware of it), it'd be nice to get user > reports, surely? > > Logging messages for libvirt might be the best that can be done there in > that case. I personally would treat that like any warning. > > > In contrast, power user tools like QMP libraries, qmp-shell and others > allow more direct and meaningful access to QMP, so those should report > deprecation messages to the user. Agreed. > >> >> [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg06131.html >> -- Thanks, Christophe de Dinechin (IRC c3d)
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
On 8/29/19 12:45 PM, Christophe de Dinechin wrote: > > Markus Armbruster writes: > >> Peter Krempa 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@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, I started replying to this thread to the other mail you sent; I think this is going to be fairly involved. I wouldn't mind being proven wrong though. > 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()? > Where's that get routed to? just an error_vprintf style situation? > 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? > How do you correlate them? > 3. Teach libvirt how to consume that new tag and pass it along? > I think it's not libvirt's job to pass it along, exactly -- libvirt made the decision for which features to engage in QEMU, not the end user. If the user upgrades QEMU but not libvirt, it's not really anything they have control over and they shouldn't be pestered with such things. However, if libvirt accidentally released a version that engages deprecated behavior (and were unaware of it), it'd be nice to get user reports, surely? Logging messages for libvirt might be the best that can be done there in that case. In contrast, power user tools like QMP libraries, qmp-shell and others allow more direct and meaningful access to QMP, so those should report deprecation messages to the user. > > [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg06131.html > > > -- > Cheers, > Christophe de Dinechin (IRC c3d) >
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
Markus Armbruster writes: > Peter Krempa 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@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()? 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? 3. Teach libvirt how to consume that new tag and pass it along? [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg06131.html -- Cheers, Christophe de Dinechin (IRC c3d)
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
Peter Krempa writes:
> On Thu, Aug 15, 2019 at 12:49:28 +0200, Kevin Wolf wrote:
>> Am 14.08.2019 um 21:27 hat John Snow geschrieben:
>
> [...]
>
>> > example:
>> >
>> > { "return": {},
>> > "deprecated": True,
>> > "warning": "Omitting filter-node-name parameter is deprecated, it will
>> > be required in the future"
>> > }
>> >
>> > There's no "error" key, so this should be recognized as success by
>> > compatible clients, but they'll definitely see the extra information.
>> >
>> > Part of my motivation is to facilitate a more aggressive deprecation of
>> > legacy features by ensuring that we are able to rigorously notify users
>> > through any means that they need to adjust their scripts.
>>
>> Who would read this, though? In the best case it ends up deep in a
>> libvirt log that nobody will look at because there was no error. In the
>> more common case, the debug level is configured so that QMP traffic
>> isn't even logged.
>
> The best we could do here is to log a warning. Thankfully we have one
> central function which always checks the returned JSON from qemu so we
> could do that universally.
>
> The would end up in the system log and alternatively also in the VM
> log file. I agree with Kevin that the possibility of it being noticed
> is rather small.
>
> 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@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.
Re: [Qemu-devel] [libvirt] [PATCH 2/2] qapi: deprecate implicit filters
On Thu, Aug 15, 2019 at 12:49:28 +0200, Kevin Wolf wrote:
> Am 14.08.2019 um 21:27 hat John Snow geschrieben:
[...]
> > example:
> >
> > { "return": {},
> > "deprecated": True,
> > "warning": "Omitting filter-node-name parameter is deprecated, it will
> > be required in the future"
> > }
> >
> > There's no "error" key, so this should be recognized as success by
> > compatible clients, but they'll definitely see the extra information.
> >
> > Part of my motivation is to facilitate a more aggressive deprecation of
> > legacy features by ensuring that we are able to rigorously notify users
> > through any means that they need to adjust their scripts.
>
> Who would read this, though? In the best case it ends up deep in a
> libvirt log that nobody will look at because there was no error. In the
> more common case, the debug level is configured so that QMP traffic
> isn't even logged.
The best we could do here is to log a warning. Thankfully we have one
central function which always checks the returned JSON from qemu so we
could do that universally.
The would end up in the system log and alternatively also in the VM
log file. I agree with Kevin that the possibility of it being noticed
is rather small.
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.
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