[PATCH V3 04/14] virtio-blk: limit max allowed submit queues

[Ming Lei]

Take blk-mq's knowledge into account for calculating io queues.

Fix wrong queue mapping in case of kdump kernel.

On arm and ppc64, 'maxcpus=1' is passed to kdump command line, see
`Documentation/admin-guide/kdump/kdump.rst`, so num_possible_cpus()
still returns all CPUs because 'maxcpus=1' just bring up one single
cpu core during booting.

blk-mq sees single queue in kdump kernel, and in driver's viewpoint
there are still multiple queues, this inconsistency causes driver to apply
wrong queue mapping for handling IO, and IO timeout is triggered.

Meantime, single queue makes much less resource utilization, and reduce
risk of kernel failure.

Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Jason Wang <jasowang at redhat.com>
Cc: virtualization at lists.linux-foundation.org
Signed-off-by: Ming Lei <ming.lei at redhat.com>
---
 drivers/block/virtio_blk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 1fe011676d07..4ba79fe2a1b4 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -1047,7 +1047,8 @@ static int init_vq(struct virtio_blk *vblk)
 
 	num_poll_vqs = min_t(unsigned int, poll_queues, num_vqs - 1);
 
-	vblk->io_queues[HCTX_TYPE_DEFAULT] = num_vqs - num_poll_vqs;
+	vblk->io_queues[HCTX_TYPE_DEFAULT] = min_t(unsigned,
+			num_vqs - num_poll_vqs, blk_mq_max_nr_hw_queues());
 	vblk->io_queues[HCTX_TYPE_READ] = 0;
 	vblk->io_queues[HCTX_TYPE_POLL] = num_poll_vqs;
 
-- 
2.40.1