virtio_ring: fix avail_wrap_counter in virtqueue_add_packed
[ Upstream commit 1acfe2c1 ] 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> Signed-off-by: Sasha Levin <sashal@kernel.org>
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