virtblk_read_limits() takes max_segments from the device's seg_max and only clamps it to VIRTIO_BLK_MAX_SG_ELEMS - 2, which fits the largest virtqueue the spec allows, not the one the device has. Without VIRTIO_RING_F_INDIRECT_DESC every segment takes a descriptor in the ring, next to the header and the status. A device that offers a seg_max above its queue size minus two thus lets the block layer build requests that can never be added: virtqueue_add_split() warns that the request is larger than the ring and returns -ENOSPC, and the hardware queue is stopped to wait for descriptors that even an empty ring does not have. The I/O hangs.
Seen with a device that offers seg_max 254 on a 128-entry queue without indirect descriptors, where the boot hangs loading /sbin/init: WARNING: drivers/virtio/virtio_ring.c:688 at virtqueue_add_split+0x6c0/0x850 ... virtqueue_add_sgs+0xd1/0x120 virtblk_add_req+0xe2/0x140 virtio_queue_rq+0x79/0x1a0 With seg_max 126 it boots, with 127 it hangs. Without indirect descriptors, clamp max_segments to the size of the smallest virtqueue minus two. The previous commit makes the probe fail for a virtqueue smaller than three, so at least one segment is left. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Stjepan Poljak <[email protected]> --- drivers/block/virtio_blk.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c index 0a2af22f2365..5f05aaa079c2 100644 --- a/drivers/block/virtio_blk.c +++ b/drivers/block/virtio_blk.c @@ -1280,6 +1280,16 @@ static int virtblk_read_limits(struct virtio_blk *vblk, /* Prevent integer overflows and honor max vq size */ sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2); + /* + * Without indirect descriptors a request takes one ring descriptor + * per segment, plus one for the header and one for the status, so + * it has to fit in the smallest of the virtqueues. A larger request + * would never fit and would wait for ring space forever. + * virtblk_probe() made sure every virtqueue holds at least three. + */ + if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC)) + sg_elems = min(sg_elems, virtblk_min_vring_size(vblk) - 2); + /* We can handle whatever the host told us to handle. */ lim->max_segments = sg_elems; -- 2.53.0
