Re: [Qemu-devel] [RFC PATCH RDMA support v5: 03/12] comprehensive protocol documentation

[Michael R. Hines]

On 04/14/2013 10:09 AM, Paolo Bonzini wrote:
> Il 14/04/2013 13:59, Michael S. Tsirkin ha scritto:
> > > I agree assuming guest has lots of zero pages won't work, but I think
> > > you are overstating the importance of overcommit.  Let's mark the damn
> > > thing as experimental, and stop making perfect the enemy of good.
> > It looks like we have to decide, before merging, whether migration with
> > rdma that breaks overcommit is worth it or not.  Since the author made
> > it very clear he does not intend to make it work with overcommit, ever.
> To me it is very much worth it.
>
> I would like to understand if unregistration would require a protocol
> change, but that's really more a curiosity than anything else.

Yes, it would require a protocol change. Either the source or the
destination would have to arbitrarily "decide" when is the time to
perform the unregistration without adversely causing the page
to be RE-registered over and over again during future iterations.

I really don't see how QEMU can accurately make such a decision.

> Perhaps it would make sense to make chunk registration permanent only
> after the bulk phase.  Chunks registered in the bulk phase are not
> permanent.

Unfortunately, that would require the entire memory footprint
to be pinned during the bulk round, which was what Michael
was originally trying to avoid a couple of weeks ago.

Nevertheless, the observation is accurate: We already have
a capability to disable chunk registration entirely.

If the user doesn't want it, they can just turn it off.


- Michael

> Paolo