[PATCH] virtio_blk: add use_irq_affinity module parameter

["Liu, Changcheng" <jerrliu@nvidia.com>]

When creating many virtio-blk devices, probe starts failing with
-ENOSPC (-28) because the system runs out of interrupt vectors:

  virtio_blk virtioNNN: probe with driver virtio_blk failed with error -28

By default virtio-blk uses managed IRQ affinity, which reserves an
interrupt vector on every CPU for each device (about nr_cpus vectors per
device). On a host with many CPUs and many devices this exhausts the
vectors long before all devices are probed.

Add use_irq_affinity (default true, no behaviour change). Set it to 0 to
use unmanaged interrupts, so each device only uses a couple of vectors
instead of one per CPU, allowing far more devices to probe.

Signed-off-by: Liu, Changcheng <jerrliu@nvidia.com>

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 32bf3ba07a9d..184f1c1f4485 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -41,6 +41,11 @@ static unsigned int poll_queues;
 module_param(poll_queues, uint, 0644);
 MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
 
+static bool use_irq_affinity = true;
+module_param(use_irq_affinity, bool, 0644);
+MODULE_PARM_DESC(use_irq_affinity,
+		 "Use managed IRQ affinity for virtqueues (default: true)");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -1016,7 +1021,8 @@ static int init_vq(struct virtio_blk *vblk)
 	}
 
 	/* Discover virtqueues and write information to configuration.  */
-	err = virtio_find_vqs(vdev, num_vqs, vqs, vqs_info, &desc);
+	err = virtio_find_vqs(vdev, num_vqs, vqs, vqs_info,
+			      use_irq_affinity ? &desc : NULL);
 	if (err)
 		goto out;
 
-- 
2.43.7