Re: [Qemu-devel] [PATCH] RDMA: Reduce restriction on block length match

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

* Juan Quintela (quintela@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > * Juan Quintela (quintela@redhat.com) wrote:
> >> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> >> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >> >
> >> > My e4d633207 patch has an over zealous sanity check that checked
> >> > the lengths of the RAM Blocks on source/destination were the same.  This
> >> > isn't true because of the 'used_length' trick for RAM blocks like the
> >> > ACPI table that vary in size.
> >> >
> >> > Prior to that patch RDMA would also fail in this case, but it should
> >> > now work with the changes in the set e4d633207 is in.
> >> >
> >> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >> >
> >> > Fixes: e4d633207c129dc5b7d145240ac4a1997ef3902f
> >> > ---
> >> >  migration/rdma.c | 13 +++++++------
> >> >  1 file changed, 7 insertions(+), 6 deletions(-)
> >> >
> >> > diff --git a/migration/rdma.c b/migration/rdma.c
> >> > index f106b2a..1d094b0 100644
> >> > --- a/migration/rdma.c
> >> > +++ b/migration/rdma.c
> >> > @@ -3338,14 +3338,15 @@ static int qemu_rdma_registration_stop(QEMUFile *f, void *opaque,
> >> >          for (i = 0; i < nb_dest_blocks; i++) {
> >> >              network_to_dest_block(&rdma->dest_blocks[i]);
> >> >  
> >> > -            /* We require that the blocks are in the same order */
> >> > +            /* We require that the blocks are in the same order,
> >> > +             * but the used_length trick for acpi blocks means that
> >> > +             * the destination can validly be larger than the source
> >> > +             */
> >> >              if (rdma->dest_blocks[i].length != local->block[i].length) {
> >> 
> >> Should we change the check to be that destination is bigger or equal
> >> than source?
> >> 
> >> With your change, we only remove the check?
> >
> > I'm actually going to drop this change; so keep the error if they're
> > different.
> >
> > My argument works like this (I've not yet found a good way to test it):
> >
> >    1) The source sends to the destination a list of RAM blocks in the qemu-file stream
> >    2) The destination performs a resize on the RAM blocks to match the source
> >        so at this point the destination's block sizes should match.
> 
> Humm, I *thought* that what the resize does is getting it bigger, but if
> destination is bigger, it does nothing, no?

The code in migration/ram.c calls qemu_ram_resize  for length != block->used_length
and similalry qemu_ram_resize always seems to set block->used_length.
I'm going to have to have more of a dig into this and figure out what's going on.

Dave

> 
> 
> >    3) The source sends a series of RDMA block registration requests for the RAM
> >    4) The destination sends a list of RAM registrations back to the source
> >    5) This check is checking that this destination list matches the local list
> >    6) As long as (4) happens after (2) then the size that the destination sees
> >       should always match the source.
> >    7) I think 4 is after 2 due to a qemu_fflush
> >
> > So keeping this check guards against 7 not really being true and/or
> > the destination populating it's list of blocks prior to (2) - which I have
> > a sneaky feeling might be happening, but am not sure yet.
> 
> 
> 
> >
> > Dave
> >
> >> 
> >> Thanks, Juan.
> >> 
> >> 
> >> > -                ERROR(errp, "Block %s/%d has a different length %" PRIu64
> >> > -                            "vs %" PRIu64, local->block[i].block_name, i,
> >> > -                            local->block[i].length,
> >> > +                fprintf(stderr, "INFO: Block %s/%d has a different length %"
> >> > +                            PRIu64 "vs %" PRIu64, local->block[i].block_name,
> >> > +                            i, local->block[i].length,
> >> >                              rdma->dest_blocks[i].length);
> >> > -                rdma->error_state = -EINVAL;
> >> > -                return -EINVAL;
> >> >              }
> >> >              local->block[i].remote_host_addr =
> >> >                      rdma->dest_blocks[i].remote_host_addr;
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK