Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling

[Daniel P . Berrangé]

On Fri, Mar 29, 2024 at 11:28:54AM +0100, Philippe Mathieu-Daudé wrote:
> Hi Zhijian,
> 
> On 29/3/24 02:53, Zhijian Li (Fujitsu) wrote:
> > 
> > 
> > On 28/03/2024 23:01, Peter Xu wrote:
> > > On Thu, Mar 28, 2024 at 11:18:04AM -0300, Fabiano Rosas wrote:
> > > > Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> > > > 
> > > > > The whole RDMA subsystem was deprecated in commit e9a54265f5
> > > > > ("hw/rdma: Deprecate the pvrdma device and the rdma subsystem")
> > > > > released in v8.2.
> > > > > 
> > > > > Remove:
> > > > >    - RDMA handling from migration
> > > > >    - dependencies on libibumad, libibverbs and librdmacm
> > > > > 
> > > > > Keep the RAM_SAVE_FLAG_HOOK definition since it might appears
> > > > > in old migration streams.
> > > > > 
> > > > > Cc: Peter Xu <peterx@redhat.com>
> > > > > Cc: Li Zhijian <lizhijian@fujitsu.com>
> > > > > Acked-by: Fabiano Rosas <farosas@suse.de>
> > > > > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> > > > 
> > > > Just to be clear, because people raised the point in the last version,
> > > > the first link in the deprecation commit links to a thread comprising
> > > > entirely of rdma migration patches. I don't see any ambiguity on whether
> > > > the deprecation was intended to include migration. There's even an ack
> > > > from Juan.
> > > 
> > > Yes I remember that's the plan.
> > > 
> > > > 
> > > > So on the basis of not reverting the previous maintainer's decision, my
> > > > Ack stands here.
> > > > 
> > > > We also had pretty obvious bugs ([1], [2]) in the past that would have
> > > > been caught if we had any kind of testing for the feature, so I can't
> > > > even say this thing works currently.
> > > > 
> > > > @Peter Xu, @Li Zhijian, what are your thoughts on this?
> > > 
> > > Generally I definitely agree with such a removal sooner or later, as 
> > > that's
> > > how deprecation works, and even after Juan's left I'm not aware of any
> > > other new RDMA users.  Personally, I'd slightly prefer postponing it one
> > > more release which might help a bit of our downstream maintenance, however
> > > I assume that's not a blocker either, as I think we can also manage it.
> > > 
> > > IMHO it's more important to know whether there are still users and whether
> > > they would still like to see it around. That's also one thing I notice 
> > > that
> > > e9a54265f533f didn't yet get acks from RDMA users that we are aware, even
> > > if they're rare. According to [2] it could be that such user may only rely
> > > on the release versions of QEMU when it broke things.
> > > 
> > > So I'm copying Yu too (while Zhijian is already in the loop), just in case
> > > someone would like to stand up and speak.
> > 
> > 
> > I admit RDMA migration was lack of testing(unit/CI test), which led to the 
> > a few
> > obvious bugs being noticed too late.
> > However I was a bit surprised when I saw the removal of the RDMA migration. 
> > I wasn't
> > aware that this feature has not been marked as deprecated(at least there is 
> > no
> > prompt to end-user).
> > 
> > 
> > > IMHO it's more important to know whether there are still users and whether
> > > they would still like to see it around.
> > 
> > Agree.
> > I didn't immediately express my opinion in V1 because I'm also consulting 
> > our
> > customers for this feature in the future.
> > 
> > Personally, I agree with Perter's idea that "I'd slightly prefer postponing 
> > it one
> > more release which might help a bit of our downstream maintenance"
> 
> Do you mind posting a deprecation patch to clarify the situation?

The key thing the first deprecation patch missed was that it failed
to issue a warning message when RDMA migration was actually used.

With regards,
Daniel
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