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    Subject[PATCH 2/2] virtio: virtio_scsi: Set can_queue to the length of the virtqueue.
    Date
    Since switching to blk-mq as the default in commit 5c279bd9e406
    ("scsi: default to scsi-mq"), virtio-scsi LUNs consume about 10x as
    much kernel memory.

    qemu currently allocates a fixed 128 entry virtqueue. can_queue
    currently is set to 1024. But with indirect descriptors, each command
    in the queue takes 1 virtqueue entry, so the number of commands which
    can be queued is equal to the length of the virtqueue.

    Note I intend to send a patch to qemu to allow the virtqueue size to
    be configured from the qemu command line.

    Thanks Paolo Bonzini, Christoph Hellwig.

    Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
    ---
    drivers/scsi/virtio_scsi.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
    index 9be211d68b15..d6b4ff634c0d 100644
    --- a/drivers/scsi/virtio_scsi.c
    +++ b/drivers/scsi/virtio_scsi.c
    @@ -972,6 +972,8 @@ static int virtscsi_probe(struct virtio_device *vdev)
    if (err)
    goto virtscsi_init_failed;

    + shost->can_queue = virtqueue_get_vring_size(vscsi->req_vqs[0].vq);
    +
    cmd_per_lun = virtscsi_config_get(vdev, cmd_per_lun) ?: 1;
    shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue);
    shost->max_sectors = virtscsi_config_get(vdev, max_sectors) ?: 0xFFFF;
    --
    2.13.1
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