Re: [Qemu-devel] [PATCH] Peek dont read for vmdescription

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Alexander Graf (agraf@suse.de) wrote:
> 
> 
> On 22.06.15 16:49, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > The VMDescription section maybe after the EOF mark, the current code
> > does a 'qemu_get_byte' and either gets the header byte identifying the
> > description or an error (which it ignores).  Doing the 'get' upsets
> > RDMA which hangs on old machine types without the VMDescription.
> > 
> > Using 'qemu_peek_byte' avoids that.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Fun. I did actually use peek at first and then figured it's the same as
> read in the qemu file implementation. Have you figured out why exactly
> peek does make a difference for the RDMA case?

Hmm, no, I agree that is odd but I do need to look again at it.

I started off with a simple empty VM (no guest running) and found that
it wouldn't migrate over RDMA using machine types older than 2.3
unless I sent the vmdesc, and this seems to fix that for me.

However, I've found another case; a busy migrate running stressapp
that still fails on older machine types.  My guess is that it depends
whether we've read the data already - if we're lucky and the data
is already in the qemu-file buffer it doesn't end up calling the RDMA
code again.

Dave


> 
> 
> Alex
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK