On Thu, 19 Sep 2024 16:32:45 +0800, Xuan Zhuo [off-list ref] wrote:

On Thu, 19 Sep 2024 16:13:51 +0800, Wenbo Li [off-list ref] wrote:

quoted

Currently, the virtio-net driver will perform a pre-dma-mapping for
small or mergeable RX buffer. But for small packets, a mismatched address
without VIRTNET_RX_PAD and xdp_headroom is used for unmapping.

That will result in unsynchronized buffers when SWIOTLB is enabled, for
example, when running as a TDX guest.

This patch unifies the address passed to the virtio core as the address of
the virtnet header and fixes the mismatched buffer address.

Changes from v2: unify the buf that passed to the virtio core in small
and merge mode.
Changes from v1: Use ctx to get xdp_headroom.

Fixes: 295525e29a5b ("virtio_net: merge dma operations when filling mergeable buffers")
Signed-off-by: Wenbo Li <redacted>
Signed-off-by: Jiahui Cen <redacted>
Signed-off-by: Ying Fang <redacted>

Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

For net/virtio maintainers:

Because the premapped mode is closed by default for virtio-net rx. So this bug
will not be triggered. DO NOT worry about it.

So that is ok if we merge it into next version. (The merge window is closed).

Thanks.

Thanks.

quoted

---
 drivers/net/virtio_net.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6f4781ec2b36..f8131f92a392 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1807,6 +1807,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
 	struct page *page = virt_to_head_page(buf);
 	struct sk_buff *skb;

+	/* We passed the address of virtnet header to virtio-core,
+	 * so truncate the padding.
+	 */
+	buf -= VIRTNET_RX_PAD + xdp_headroom;
+
 	len -= vi->hdr_len;
 	u64_stats_add(&stats->bytes, len);

@@ -2422,8 +2427,9 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
 	if (unlikely(!buf))
 		return -ENOMEM;

-	virtnet_rq_init_one_sg(rq, buf + VIRTNET_RX_PAD + xdp_headroom,
-			       vi->hdr_len + GOOD_PACKET_LEN);
+	buf += VIRTNET_RX_PAD + xdp_headroom;
+
+	virtnet_rq_init_one_sg(rq, buf, vi->hdr_len + GOOD_PACKET_LEN);

 	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
 	if (err < 0) {
--

2.20.1