virtio-blk: clamp max_segments to virtqueue ring size

In virtblk_add_req(), each request consumes 2 extra descriptors (out_hdr
and in_hdr) in addition to the data scatter-gather segments.

When a hypervisor (e.g. QNX Hypervisor) advertises VIRTIO_BLK_F_SEG_MAX
with seg_max = 1024 alongside a 1024-entry split ring (vring.num = 1024)
and VIRTIO_RING_F_INDIRECT_DESC disabled, lim->max_segments is set to
1024. When the block layer submits requests with 1023 or 1024 data
segments, total_sg reaches 1025 or 1026. This exceeds vring.num (1024),
causing virtqueue_add_split() to return -ENOSPC and permanently wedge
the blk-mq queue.

Furthermore, the Virtio specification (2.7.5.3.1) requires that a
descriptor chain never exceed the Queue Size, and virtqueue_add_split()
falls back to direct descriptors if indirect table allocation fails.

Fix this by:
1. Rejecting queues with ring_size < 3 at probe, or
   ring_size < (queue_max_segments + 2) during resume/reset recovery in
   init_vq().
2. Unconditionally clamping sg_elems to (ring_size - 2) across all
   virtqueues in virtblk_read_limits().

Fixes: 0864b79a1533 ("virtio: block: dynamic maximum segments")
Signed-off-by: Sergii Ushakov <sergiiushakov@google.com>
Link: https://patch.msgid.link/20260918135120.1261150-1-sergiiushakov@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions