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Subject[PATCH 2/2] scsi: virtio_scsi: limit number of hw queues by nr_cpu_ids
When tag_set->nr_maps is 1, the block layer limits the number of hw queues
by nr_cpu_ids. No matter how many hw queues are used by virtio-scsi, as it
has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.

In addition, specifically for pci scenario, when the 'num_queues' specified
by qemu is more than maxcpus, virtio-scsi would not be able to allocate
more than maxcpus vectors in order to have a vector for each queue. As a
result, it falls back into MSI-X with one vector for config and one shared
for queues.

Considering above reasons, this patch limits the number of hw queues used
by virtio-scsi by nr_cpu_ids.

Signed-off-by: Dongli Zhang <>
drivers/scsi/virtio_scsi.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 8af0177..9c4a3e1 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -793,6 +793,7 @@ static int virtscsi_probe(struct virtio_device *vdev)

/* We need to know how many queues before we allocate. */
num_queues = virtscsi_config_get(vdev, num_queues) ? : 1;
+ num_queues = min_t(unsigned int, nr_cpu_ids, num_queues);

num_targets = virtscsi_config_get(vdev, max_target) + 1;

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