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

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Greg Kurz <groug@kaod.org>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <fam@euphon.net>,
	qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [for-6.1 2/4] virtio-blk: Configure all host notifiers in a single MR transaction
Date: Wed, 5 May 2021 11:14:23 +0100	[thread overview]
Message-ID: <YJJv/5jWOwEu92C/@stefanha-x1.localdomain> (raw)
In-Reply-To: <20210407143501.244343-3-groug@kaod.org>

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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.

> +
>      /* 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.

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