Re: [PATCH v2] virtio_net: Fix mismatched buf address when unmapping for small packets

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed,  4 Sep 2024 14:10:09 +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.

Will used virt_to_head_page(), so could you say more about it?

	struct page *page = virt_to_head_page(buf);

Thanks.

>
> That will result in unsynchronized buffers when SWIOTLB is enabled, for
> example, when running as a TDX guest.
>
> This patch handles small and mergeable packets separately and fixes
> the mismatched buffer address.
>
> 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 | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c6af18948..cbc3c0ae4 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -891,6 +891,23 @@ static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx)
>  	return buf;
>  }
>
> +static void *virtnet_rq_get_buf_small(struct receive_queue *rq,
> +				      u32 *len,
> +				      void **ctx,
> +				      unsigned int header_offset)
> +{
> +	void *buf;
> +	unsigned int xdp_headroom;
> +
> +	buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
> +	if (buf) {
> +		xdp_headroom = (unsigned long)*ctx;
> +		virtnet_rq_unmap(rq, buf + VIRTNET_RX_PAD + xdp_headroom, *len);
> +	}
> +
> +	return buf;
> +}
> +
>  static void virtnet_rq_init_one_sg(struct receive_queue *rq, void *buf, u32 len)
>  {
>  	struct virtnet_rq_dma *dma;
> @@ -2692,13 +2709,23 @@ static int virtnet_receive_packets(struct virtnet_info *vi,
>  	int packets = 0;
>  	void *buf;
>
> -	if (!vi->big_packets || vi->mergeable_rx_bufs) {
> +	if (vi->mergeable_rx_bufs) {
>  		void *ctx;
>  		while (packets < budget &&
>  		       (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
>  			receive_buf(vi, rq, buf, len, ctx, xdp_xmit, stats);
>  			packets++;
>  		}
> +	} else if (!vi->big_packets) {
> +		void *ctx;
> +		unsigned int xdp_headroom = virtnet_get_headroom(vi);
> +		unsigned int header_offset = VIRTNET_RX_PAD + xdp_headroom;
> +
> +		while (packets < budget &&
> +		       (buf = virtnet_rq_get_buf_small(rq, &len, &ctx, header_offset))) {
> +			receive_buf(vi, rq, buf, len, ctx, xdp_xmit, stats);
> +			packets++;
> +		}
>  	} else {
>  		while (packets < budget &&
>  		       (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
> --
> 2.20.1
>