Re: [Qemu-devel] [PATCH 0/3] Export machine type deprecation info through QMP

[Eduardo Habkost]

On Thu, May 09, 2019 at 05:08:11PM +0100, Daniel P. Berrangé wrote:
> On Thu, May 09, 2019 at 12:52:47PM -0300, Eduardo Habkost wrote:
> > On Thu, May 09, 2019 at 10:14:52AM +0100, Daniel P. Berrangé wrote:
> > > On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> > > > We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> > > > 
> > > > Eduardo Habkost <address@hidden> writes:
> > > > 
> > > > > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> > > > >> Eduardo Habkost <address@hidden> writes:
> > > > >> 
> > > > >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> > > > >> >> Eduardo Habkost <address@hidden> writes:
> > > > >> >> 
> > > > >> >> > This series adds machine type deprecation information to the
> > > > >> >> > output of the `query-machines` QMP command.  With this, libvirt
> > > > >> >> > and management software will be able to show this information to
> > > > >> >> > users and/or suggest changes to VM configuration to avoid
> > > > >> >> > deprecated machine types.
> > > > >> >> 
> > > > >> >> This overlaps with something I want to try, namely using Kevin's
> > > > >> >> proposed QAPI feature flags for deprecation markings.  Let's 
> > > > >> >> compare the
> > > > >> >> two.
> > > > >> >> 
> > > > >> >> To mark something as deprecated with your patches, you add a
> > > > >> >> @support-status member somewhere, where "somewhere" is related to
> > > > >> >> "something" by "provides information on".
> > > > >> >> 
> > > > >> >> Example: MachineInfo (returned by query-machines) provides 
> > > > >> >> information
> > > > >> >> on possible values of -machine parameter type.  If -machine was
> > > > >> >> QAPIfied, it would provide information on possible values of a 
> > > > >> >> QAPI
> > > > >> >> object type's member.  The type might be anonymous.  The member 
> > > > >> >> should
> > > > >> >> be an enum (we currently use 'str' in MachineInfo).
> > > > >> >
> > > > >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> > > > >> >
> > > > >> >> 
> > > > >> >> Example: say we want to deprecate block driver "vfat",
> > > > >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> > > > >> >> multiple places; let's ignore all but BlockdevOptions.  We need 
> > > > >> >> to add
> > > > >> >> @support-status to something that provides information on
> > > > >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc 
> > > > >> >> query
> > > > >> >> providing information on either of the two, because QAPI/QMP
> > > > >> >> introspection has been sufficient.  What now?
> > > > >> >> 
> > > > >> >> Can we add deprecation information to (general) QAPI/QMP 
> > > > >> >> introspection
> > > > >> >
> > > > >> > Yes, we can.  I think it's a good idea.  But:
> > > > >> >
> > > > >> >> instead of ad hoc queries?
> > > > >> >
> > > > >> > I'm not sure about the "instead of" part.  I don't want perfect
> > > > >> > to be the enemy of done, and I don't want QAPIfication of
> > > > >> > -machine to be a requirement to start reporting machine type
> > > > >> > deprecation information.
> > > > >> 
> > > > >> Valid point.  Still, I believe we should at least try to predict how 
> > > > >> the
> > > > >> pieces we create now would fit with the pieces we plan to create 
> > > > >> later
> > > > >> on.
> > > > >
> > > > > Sure.
> > > > >
> > > > >> 
> > > > >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> > > > >> feature "deprecated" work for machine types.  Turning MachineInfo 
> > > > >> member
> > > > >> @name into an enum, so we can tack "deprecated" onto its values, 
> > > > >> would
> > > > >> suffice.
> > > > >> 
> > > > >> Such a QAPIfication of machine types is still hard: QOM types are
> > > > >> defined at compile time just like the QAPI schema, but their 
> > > > >> definition
> > > > >> is distributed, and collected into one place only at run time.  I
> > > > >> discussed this on slide 39 of my "QEMU interface introspection: From
> > > > >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but 
> > > > >> it's
> > > > >> just a special case of QOM.  Choices listed there:
> > > > >> 
> > > > >> * Collect drivers at compile time? Hard...
> > > > >> * Make QAPI schema dynamic? Hard...
> > > > >> * Forgo driver-specific arguments in schema?
> > > > >>   Defeats introspection...
> > > > >> 
> > > > >> I'd like to add to the last item:
> > > > >> 
> > > > >>   Provide QOM introspection on par with QAPI schema introspection
> > > > >> 
> > > > >> The QOM introspection we have (qom-list-types etc. is not on par.
> > > > >
> > > > > Agreed, but do we really want to do it?  We have been avoiding
> > > > > exposing QOM internals to the outside on purpose.  I believe
> > > > > there are at least two reasons for that:
> > > > >
> > > > > 1) Not every QOM type/property is supposed to be visible to the
> > > > >    outside
> > > > 
> > > > True.
> > > > 
> > > > However, the parts of QOM exposed via device_add and object-add are
> > > > definitely part of the stable external interface (unless explicitly
> > > > marked unstable).
> > > > 
> > > > >            (and nobody really knows what's the full set of
> > > > >    supported external QOM interfaces);
> > > > 
> > > > Also true.  And terrible.
> > > > 
> > > > > 2) QAPI is our preferred interface interface 
> > > > > specification/introspection
> > > > >    mechanism.
> > > > 
> > > > When preferences and requirements collide, preferences tend to get run
> > > > over.
> > > > 
> > > > The QAPI schema is *declarative*: the schema declares QAPI objects and
> > > > properties.  We generate C from the schema, which we then compile and
> > > > link into QEMU.
> > > > 
> > > > QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> > > > define QOM objects and properties.  Maximizes flexibility.  See also
> > > > Turing tarpit.
> > > > 
> > > > No matter how much we'd prefer to use QAPI to specify external
> > > > interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> > > > much more static.  Either is hard.  Quite possibly infeasible.
> > > > 
> > > > We could try to extend QAPI/QMP introspection to somehow merge in
> > > > additional information at run-time[1].  Could be regarded as a limited
> > > > way to make QAPI more dynamic.  This is in the "vague idea, not sure
> > > > it's feasible" stage.
> > > > 
> > > > We could try to rearchitect QOM so that you can optionally specify QOM
> > > > stuff in the QAPI schema, then require that for QOM stuff that's part of
> > > > a stable external interface.  Same "vague idea" stage, feasibility even
> > > > more doubtful.
> > > 
> > > I think it is feasible to use QAPI to declare the QOM object type,
> > > QOM parent type, list of QOM interfaces and QOM properties in a
> > > declarative manner. From that generate all the tedious boilerplate
> > > code. The dev then just has to provide the "interesting" code for
> > > the object.
> > > 
> > > I'd really like to do a PoC of this but never have free time :-(
> > > Perhaps an interesting task for someone who wants to delve into
> > > some new work...
> > > 
> > > 
> > > > >> >> Example: to deprecate block driver "vfat", add feature 
> > > > >> >> "deprecated" to
> > > > >> >> BlockdevDriver member @vfat.
> > > > >> >> 
> > > > >> >> Unlike your patches, this does not require finding a "somewhere" 
> > > > >> >> that
> > > > >> >> provides information on "something".  You simply tack 
> > > > >> >> "deprecated" right
> > > > >> >> onto "something".
> > > > >> >> 
> > > > >> >> Your patches provide more information, however: human-readable 
> > > > >> >> messages.
> > > > >> >
> > > > >> > It also includes a machine-friendly suggested alternative (which
> > > > >> > I think is even more important that the human-readable message).
> > > > >> 
> > > > >> I agree we should point to a preferred replacement whenever we 
> > > > >> deprecate
> > > > >> something.
> > > > >> 
> > > > >> We have to do it in documentation.  And we generally do, in
> > > > >> qemu-deprecated.texi.
> > > > >> 
> > > > >> How useful would doing it in QMP as well be?  Depends on what 
> > > > >> management
> > > > >> applications can do with the additional information.
> > > > >
> > > > > I expect it to be useful for things that have obvious
> > > > > replacements, like old machine type or CPU model versions.
> > > > 
> > > > I doubt a management application should apply suggested replacements
> > > > automatically, and I doubt libvirt would.  Not even when QEMU developers
> > > > deem them "obvious".
> > > 
> > > We certainly won't apply the suggested replacement as in many cases
> > > it is not going to be a functionally equivalent drop-in.
> > 
> > Who's "we"?
> 
> I was refering to libvirt here.
> 
> > > If QEMU logs it to stderr, it will end up in the per-VM log file
> > > libvirt has under /var/log/libvirt/qemu/$GUESTNAME.log.  If QEMU
> > > doesn't log it to stderr, then libvirt would just write it to
> > > that same log file itself.
> > > 
> > > If libvirt gains some API or event for notifying apps of deprecation
> > > we might bubble it up to the mgmt app that way.
> > > 
> > > I still feel it is useful to have the suggested replacement in the
> > > logs, rather  than only leaving it in qemu-deprecated.texi.  This
> > > way the info is immediately visible to both app developers and any
> > > support person dealing with bugs.
> > > 
> > > If the app dev see the suggested replacement upfront they're more
> > > likely to make an immediate decision to update their code if the
> > > suggestion is trivial. If they need to go find the QEMU docs to
> > > lookup what action is required I feel they'll more likely just
> > > put the item on their long todo list where it will languish.
> > 
> > Agreed.  However, note that the audience for deprecation
> > information is not just developers and support people.  End users
> > need to know when they are relying on a deprecated feature, and
> > applications should make it as easy as possible for them to
> > update their configurations.
> > 
> > I'm not suggesting the alternative would be applied
> > automatically.  But having the alternative available in a
> > machine-friendly way may be the difference between a unhelpful UI
> > that just tells the user there's some problem but can't give a
> > solution, and one that can really assist the user to fix the
> > problem.
> 
> For some aspects of QEMU it might be possible, but considering the
> broader set of things which can be deprecated, I don't think it is
> possible to expose a machine consumable "suggestion".
> 
> Consider the deprecation of the ACL feature. We deprecated monitor
> commands "acl_add", "acl_policy", "acl_reset", etc. The suggested
> replacement is to use one of the many possible QAuthZ types combined
> with the -object arg. Even if we invented some way to express this
> in the schema, I don't think any app would usefully consume it.

No problem, we don't need to suggest a machine consumable
alternative for everything.

I'm thinking about features that are visible to the end user and
require user action to fix their configuration, like machine type
versions or CPU model versions.

-- 
Eduardo