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

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Apr 14, 2013 at 10:27:24AM -0400, Michael R. Hines wrote:
> On 04/14/2013 07:59 AM, Michael S. Tsirkin wrote:
> >On Fri, Apr 12, 2013 at 04:43:54PM +0200, Paolo Bonzini wrote:
> >>Il 12/04/2013 13:25, Michael S. Tsirkin ha scritto:
> >>>On Fri, Apr 12, 2013 at 12:53:11PM +0200, Paolo Bonzini wrote:
> >>>>Il 12/04/2013 12:48, Michael S. Tsirkin ha scritto:
> >>>>>1.  You have two protocols already and this does not make sense in
> >>>>>version 1 of the patch.
> >>>>It makes sense if we consider it experimental (add x- in front of
> >>>>transport and capability) and would like people to play with it.
> >>>>
> >>>>Paolo
> >>>But it's not testable yet.  I see problems just reading the
> >>>documentation.  Author thinks "ulimit -l 10000000000" on both source and
> >>>destination is just fine.  This can easily crash host or cause OOM
> >>>killer to kill QEMU.  So why is there any need for extra testers?  Fix
> >>>the major bugs first.
> >>>
> >>>There's a similar issue with device assignment - we can't fix it there,
> >>>and despite being available for years, this was one of two reasons that
> >>>has kept this feature out of hands of lots of users (and assuming guest
> >>>has lots of zero pages won't work: balloon is not widely used either
> >>>since it depends on a well-behaved guest to work correctly).
> >>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.
> >>
> >>Paolo
> >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.
> >
> That depends entirely as what you define as overcommit.

You don't get to define your own terms.  Look it up in wikipedia or
something.

> 
> The pages do get unregistered at the end of the migration =)
> 
> - Michael

The limitations are pretty clear, and you really should document them:

1. run qemu as root, or under ulimit -l <total guest memory> on both source and
  destination

2. expect that as much as that amount of memory is pinned
  and unvailable to host kernel and applications for
  arbitrarily long time.
  Make sure you have much more RAM in host or QEMU will get killed.

To me, especially 1 is an unacceptable security tradeoff.
It is entirely fixable but we both have other priorities,
so it'll stay broken.

-- 
MST