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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 4 Jul 2018 15:34:40 +0200
Kevin Wolf <kwolf@redhat.com> 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.

Consider people running regression tests: They likely have a set of
machine definitions they use, some of which may include serial numbers
et al., and that suddenly breaks if they try with a newer QEMU. If they
can just update to a newer (released) libvirt, their regression tests
continue to work.

> 
> 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 has been that while a recent-ish libvirt might not
exploit the latest QEMU features, it still continues to work. But yes,
this is a good point. We need to agree on a clear policy and document
that.

> 
> > 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
> 
> 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.

I understand your concern, but not having a way out if something fell
through the cracks is hurting people testing QEMU -- IOW, people we
really don't want to hurt. Ideas on what could help:

- A clearer way to communicate to libvirt users that libvirt is using a
  deprecated API, so that they can report it and libvirt can change its
  code. The main problem is that people usually don't read logs if
  everything seems to work fine...
- A documented way in our QEMU deprecation process to hold off for one
  release, which can be used at most twice (or maybe just once?) No
  endless loops that way :)

> 
> 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.

Fully agreed on sorting out our deprecation mechanism.

I can send a revert patch, if nobody else beats me to it.

Thanks!