Re: [PATCH] migration/rdma: Fix return-path case

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

* Peter Xu (peterx@redhat.com) wrote:
> On Tue, Mar 14, 2023 at 05:15:58PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > The RDMA code has return-path handling code, but it's only enabled
> > if postcopy is enabled; if the 'return-path' migration capability
> > is enabled, the return path is NOT setup but the core migration
> > code still tries to use it and breaks.
> > 
> > Enable the RDMA return path if either postcopy or the return-path
> > capability is enabled.
> > 
> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=2063615
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> 
> > @@ -3373,7 +3373,8 @@ static int qemu_rdma_accept(RDMAContext *rdma)
> >       * initialize the RDMAContext for return path for postcopy after first
> >       * connection request reached.
> >       */
> > -    if (migrate_postcopy() && !rdma->is_return_path) {
> > +    if ((migrate_postcopy() || migrate_use_return_path())
> > +        && !rdma->is_return_path) {
> >          rdma_return_path = qemu_rdma_data_init(rdma->host_port, NULL);
> >          if (rdma_return_path == NULL) {
> >              rdma_ack_cm_event(cm_event);
> 
> It's not extremely clear to me yet on when we should use migrate_postcopy()
> and when to use migrate_postcopy_ram().  I think it's because I don't know
> enough on the dirty-bitmaps capability.  Do we have some good documentation
> somewhere?

Hmm that's probably a good point.

> Not much I get from the qapi doc..
> 
> # @dirty-bitmaps: If enabled, QEMU will migrate named dirty bitmaps.
> #                 (since 2.12)

I don't know of any good docs; I think this is a blocks mechanism; I'm
not even sure if it needs the return path.

Dave

> Thanks,
> 
> -- 
> Peter Xu
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK