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

[Greg Kurz]

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 ?

> > +
> >      /* 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.

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