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 into 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 <liwenbo.martin@bytedance.com>
Signed-off-by: Jiahui Cen <cenjiahui@bytedance.com>
Signed-off-by: Ying Fang <fangying.tommy@bytedance.com>
---
 drivers/net/virtio_net.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6f4781ec2b36..9446666c84aa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1804,9 +1804,15 @@ static struct sk_buff *receive_small(struct net_device *dev,
 				     struct virtnet_rq_stats *stats)
 {
 	unsigned int xdp_headroom = (unsigned long)ctx;
-	struct page *page = virt_to_head_page(buf);
+	struct page *page;
 	struct sk_buff *skb;
 
+	// We passed the address of virtnet header to virtio-core,
+	// so truncate the padding.
+	buf -= VIRTNET_RX_PAD + xdp_headroom;
+
+	page = virt_to_head_page(buf);
+
 	len -= vi->hdr_len;
 	u64_stats_add(&stats->bytes, len);
 
@@ -2422,8 +2428,9 @@ 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

On Thu, 19 Sep 2024 11:52:14 +0800, Wenbo Li <liwenbo.martin@bytedance.com> wrote:
> 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 into 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 <liwenbo.martin@bytedance.com>
> Signed-off-by: Jiahui Cen <cenjiahui@bytedance.com>
> Signed-off-by: Ying Fang <fangying.tommy@bytedance.com>
> ---
>  drivers/net/virtio_net.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6f4781ec2b36..9446666c84aa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1804,9 +1804,15 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  				     struct virtnet_rq_stats *stats)
>  {
>  	unsigned int xdp_headroom = (unsigned long)ctx;
> -	struct page *page = virt_to_head_page(buf);
> +	struct page *page;

Because that here is head page, so you can keep the original code.

>  	struct sk_buff *skb;
>
> +	// We passed the address of virtnet header to virtio-core,
> +	// so truncate the padding.

Please check the kernel code style.

Thanks.

> +	buf -= VIRTNET_RX_PAD + xdp_headroom;
> +
> +	page = virt_to_head_page(buf);
> +
>  	len -= vi->hdr_len;
>  	u64_stats_add(&stats->bytes, len);
>
> @@ -2422,8 +2428,9 @@ 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
>