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[PATCH 0/2] virtio_scsi: Set can_queue based on size of virtqueue.

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  [PATCH 0/2] virtio_scsi: Set can_queue based on size of virtqueue. "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-10 18:50 +0200
    [PATCH 2/2] virtio: virtio_scsi: Set can_queue to the length of the virtqueue. "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-10 18:50 +0200

#1708875 — [PATCH 0/2] virtio_scsi: Set can_queue based on size of virtqueue.

Earlier discussion:
https://lkml.org/lkml/2017/8/4/601
"Increased memory usage with scsi-mq"

Downstream bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=1478201

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#1708878 — [PATCH 2/2] virtio: virtio_scsi: Set can_queue to the length of the virtqueue.

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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