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Subject[PATCH AUTOSEL 4.19 44/53] virtio-blk: limit number of hw queues by nr_cpu_ids
From: Dongli Zhang <>

[ Upstream commit bf348f9b78d413e75bb079462751a1d86b6de36c ]

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-blk, 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-blk 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

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

Reviewed-by: Stefan Hajnoczi <>
Signed-off-by: Dongli Zhang <>
Signed-off-by: Jens Axboe <>
Signed-off-by: Sasha Levin <>
drivers/block/virtio_blk.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 23752dc99b00..dd64f586679e 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -446,6 +446,8 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;

+ num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
return -ENOMEM;
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