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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jul 04, 2018 at 03:34:40PM +0200, Kevin Wolf wrote:
> Am 04.07.2018 um 15:02 hat Cornelia Huck geschrieben:
> > On Tue, 3 Jul 2018 13:32:29 +0200
> > Kevin Wolf <kwolf@redhat.com> wrote:
> > 
> > > > > > Has serial/gemoetry been fixed meanwhile and will it make it into the
> > > > > > next release?  
> > > > > 
> > > > > I cannot find an archive that has it, but it is on the libvirt mailing
> > > > > list as "[libvirt] [PATCH v3] qemu: format serial and geometry on frontend disk device".
> > > > > Review seems done, but it has missed libvirt 4.5 which was released today.  
> > > > 
> > > > Just posted latest version here:
> > > > 
> > > >   https://www.redhat.com/archives/libvir-list/2018-July/msg00130.html
> > > > 
> > > > It will be in the next release on ~ Aug 1st  
> > > 
> > > It would have been a lot nicer to have it the July release because this
> > > means that we'll have the released libvirt broken during almost the
> > > whole rc phase of QEMU 3.0, but the release is planned for Aug 8th the
> > > earliest, so I guess we're still okay. People using QEMU from git will
> > > just need libvirt from git as well.
> > 
> > Speaking as an innocent* bystander:
> > 
> > I would usually presume that I can use any recent libvirt to test
> > current QEMU, even bleeding edge. In this case, not even the latest
> > released libvirt version will be fine, I would also need to build
> > libvirt from git (which is probably not something a non-libvirt
> > developer will usually do). If everything goes according to plan, I can
> > only test QEMU with a released libvirt version at the very tail end of
> > hardfreeze, where only release blockers are appropriate.
> 
> I understand where you're coming from, but let's be honest: It's not as
> if disk geometry or serial numbers were features that absolutely need
> to be there to give QEMU any testing.

I'd venture to suggest disk geometry is almost never used in practice.

serial strings, however, are pretty common, used by default in fact
by openstack.

> Also, my understanding has always been that we expect users to have a
> libvirt version that isn't older than QEMU. It would be useful to set a
> clear policy for this and document it.

My understanding was actually mostly the opposite. We'll try not to
/knowingly/ break existing libvirt version if reasonable. We've not
always followed that, either by accident, or even intentionally in
some cases. What's never defined is just how old libvirt we care about
not breaking. In the case of QEMU libvirt will in fact be fixed before
QEMU 3.0 is released, but only by a week or so.

I think its mostly a case of being pragmatic about what we do in this
respect, rather than defining a hard rule.

In this case I'd personally lean towards reverting this patch, since
merging it was not a critical blocker to other work that went into
3.0 IIUC. So block maintaniers burden shouldn't be impacted by a
temporary revert and then re-merge when 3.1 opens.

> > I think it would be really beneficial to general QEMU test coverage to
> > push deleting this option back a release or two. We should make testing
> > QEMU in conjunction with libvirt as uncomplicated as possible.
> 
> Essentially, what is important to me isn't getting these options dropped
> exactly in 3.0, but not setting a bad precedence that deprecation isn't
> actually worth anything. We may easily end up with this deprecation
> process:
> 
> depreate a feature
> release QEMU version n + 1
> release QEMU version n + 2
> remove the feature
> while libvirt hasn't removed use of the feature:
>     # ...and why should it when everything is still working?
>     reinstate the feature
>     release QEMU version n + x
>     remove the feature

Yeah, we definitely don't want to get into that kind of mess
in general.

> When management tools know that this is the process, the motivation to
> remove the use of the feature gets even lower (not out of malice, but
> because there will be always more important things), so this will
> "optimise" itself into an endless loop and we're back to never actually
> removing old cruft that impedes development.
> 
> The libvirt patch has just been merged (and I'm almost sure that this
> wouldn't have happened so quickly if I had just reverted the patch right
> away), so at least we know now that this specific instance of the loop
> is going to terminate.
> 
> What's left is first and foremost that we need to sort out our broken
> deprecation mechanism, and if that gets done, I don't mind if someone
> wants to revert the patch for 3.0 as long as they also take care that it
> gets back into 3.1.

What I think this really highlights is

 - Lack of libvirt paying attention to deprecations
 - Lack of a way to get good diagnostic of violations when testing

The first thing is just a human problem on libvirt side.

The second thing could be addressed in code if there was an env variable
or cli option to turn all use of deprecated features into abort()s in
QEMU. That way libvirt automated testing against git master would have
a way to identify places where we use deprecated feature.

Regards,
Daniel
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