Re: [for-6.1 2/4] virtio-blk: Configure all host notifiers in a single MR transaction

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

On Wed, May 05, 2021 at 01:15:51PM +0200, Greg Kurz wrote:
> On Wed, 5 May 2021 11:14:23 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > On Wed, Apr 07, 2021 at 04:34:59PM +0200, Greg Kurz wrote:
> > > This allows the virtio-blk-pci device to batch the setup of all its
> > > host notifiers. This significantly improves boot time of VMs with a
> > > high number of vCPUs, e.g. from 3m26.186s down to 0m58.023s for a
> > > pseries machine with 384 vCPUs.
> > > 
> > > Note that memory_region_transaction_commit() must be called before
> > > virtio_bus_cleanup_host_notifier() because the latter might close
> > > ioeventfds that the transaction still assumes to be around when it
> > > commits.
> > > 
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  hw/block/dataplane/virtio-blk.c | 25 +++++++++++++++++++++++++
> > >  1 file changed, 25 insertions(+)
> > > 
> > > diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> > > index d7b5c95d26d9..cd81893d1d01 100644
> > > --- a/hw/block/dataplane/virtio-blk.c
> > > +++ b/hw/block/dataplane/virtio-blk.c
> > > @@ -198,19 +198,30 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
> > >          goto fail_guest_notifiers;
> > >      }
> > >  
> > > +    memory_region_transaction_begin();
> > 
> > This call is optional and an optimization. It would be nice to have a
> > comment here explaining that - otherwise people may wonder why this is
> > necessary.
> > 
> 
> Indeed. Same goes for patch 4 actually.
> 
> Michael,
> 
> Should I re-post an updated version of this series or can the
> comments be added in a followup ?


An updated version is better. Stefan did not ack anyway.

> > > +
> > >      /* Set up virtqueue notify */
> > >      for (i = 0; i < nvqs; i++) {
> > >          r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, true);
> > >          if (r != 0) {
> > > +            int j = i;
> > > +
> > >              fprintf(stderr, "virtio-blk failed to set host notifier (%d)\n", r);
> > >              while (i--) {
> > >                  virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
> > > +            }
> > > +
> > > +            memory_region_transaction_commit();
> > > +
> > > +            while (j--) {
> > >                  virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
> > >              }
> > >              goto fail_host_notifiers;
> > >          }
> > >      }
> > >  
> > > +    memory_region_transaction_commit();
> > > +
> > >      s->starting = false;
> > >      vblk->dataplane_started = true;
> > >      trace_virtio_blk_data_plane_start(s);
> > > @@ -246,8 +257,15 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
> > >      return 0;
> > >  
> > >    fail_aio_context:
> > > +    memory_region_transaction_begin();
> > 
> > Probably unnecessary since this is an error code path. We don't need to
> > optimize it.
> > 
> > Doesn't hurt though.
> 
> True. I can drop this if I repost.