virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

In current packed virtqueue implementation, the avail_wrap_counter won't
flip, in the case when the driver supplies a descriptor chain with a
length equals to the queue size; total_sg == vq->packed.vring.num.

Let’s assume the following situation:
vq->packed.vring.num=4
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

Then the driver adds a descriptor chain containing 4 descriptors.

We expect the following result with avail_wrap_counter flipped:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 1

But, the current implementation gives the following result:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

To reproduce the bug, you can set a packed queue size as small as
possible, so that the driver is more likely to provide a descriptor
chain with a length equal to the packed queue size. For example, in
qemu run following commands:
sudo qemu-system-x86_64 \
-enable-kvm \
-nographic \
-kernel "path/to/kernel_image" \
-m 1G \
-drive file="path/to/rootfs",if=none,id=disk \
-device virtio-blk,drive=disk \
-drive file="path/to/disk_image",if=none,id=rwdisk \
-device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
indirect_desc=off \
-append "console=ttyS0 root=/dev/vda rw init=/bin/bash"

Inside the VM, create a directory and mount the rwdisk device on it. The
rwdisk will hang and mount operation will not complete.

This commit fixes the wrap counter error by flipping the
packed.avail_wrap_counter, when start of descriptor chain equals to the
end of descriptor chain (head == i).

Fixes: 1ce9e605 ("virtio_ring: introduce packed ring support")
Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
Message-Id: <20230808051110.3492693-1-yuanyaogoog@chromium.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -1499,7 +1499,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
 		}
 	}
 
-	if (i < head)
+	if (i <= head)
 		vq->packed.avail_wrap_counter ^= 1;
 
 	/* We're using some buffers from the free list. */