Re: [PATCH RFC 9/9] migration/rdma: Remove rdma_cm_poll_handler

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 17, 2025 at 03:38:35PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > This almost reverts commit 923709896b1b01fb982c93492ad01b233e6b6023.
> >
> > It was needed because the RDMA iochannel on dest QEMU used to only yield
> > without monitoring the fd.  Now it should be monitored by the same poll()
> > similarly on the src QEMU in qemu_rdma_wait_comp_channel().  So even
> > without the fd handler, dest QEMU should be able to receive the events.
> >
> > I tested this by initiating an RDMA migration, then do two things:
> >
> >   - Either does migrate_cancel on src, or,
> >   - Directly kill destination QEMU
> >
> > In both cases, the other side of QEMU will be able to receive the
> > disconnect event in qemu_rdma_wait_comp_channel() and properly cancel or
> > fail the migration.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/rdma.c | 29 +----------------------------
> >  1 file changed, 1 insertion(+), 28 deletions(-)
> >
> > diff --git a/migration/rdma.c b/migration/rdma.c
> > index 7751262460..da7fd48bf3 100644
> > --- a/migration/rdma.c
> > +++ b/migration/rdma.c
> > @@ -3045,32 +3045,6 @@ int rdma_control_save_page(QEMUFile *f, ram_addr_t block_offset,
> >  
> >  static void rdma_accept_incoming_migration(void *opaque);
> >  
> > -static void rdma_cm_poll_handler(void *opaque)
> > -{
> > -    RDMAContext *rdma = opaque;
> > -    struct rdma_cm_event *cm_event;
> > -
> > -    if (rdma_get_cm_event(rdma->channel, &cm_event) < 0) {
> > -        error_report("get_cm_event failed %d", errno);
> > -        return;
> > -    }
> > -
> > -    if (cm_event->event == RDMA_CM_EVENT_DISCONNECTED ||
> > -        cm_event->event == RDMA_CM_EVENT_DEVICE_REMOVAL) {
> > -        if (!rdma->errored &&
> > -            migration_incoming_get_current()->state !=
> > -              MIGRATION_STATUS_COMPLETED) {
> > -            error_report("receive cm event, cm event is %d", cm_event->event);
> > -            rdma->errored = true;
> > -            if (rdma->return_path) {
> > -                rdma->return_path->errored = true;
> > -            }
> > -        }
> > -        rdma_ack_cm_event(cm_event);
> > -    }
> > -    rdma_ack_cm_event(cm_event);
> > -}
> > -
> >  static int qemu_rdma_accept(RDMAContext *rdma)
> >  {
> >      Error *err = NULL;
> > @@ -3188,8 +3162,7 @@ static int qemu_rdma_accept(RDMAContext *rdma)
> >                              NULL,
> >                              (void *)(intptr_t)rdma->return_path);
> >      } else {
> > -        qemu_set_fd_handler(rdma->channel->fd, rdma_cm_poll_handler,
> > -                            NULL, rdma);
> > +        qemu_set_fd_handler(rdma->channel->fd, NULL, NULL, NULL);
> 
> I'm not familiar with this code, but is this left here to remove the
> handler? Can't we remove this line altogether?

Fair question. I was just lazy because I know it's safe to call it like
that no matter what, unregistering anything if we registered some,
otherwise this qemu_set_fd_handler() should be a no-op.

I am just not confident on RDMA code that we can remove it.  IOW, before
923709896b1 we did that, so I kept it as-is.

-- 
Peter Xu