Re: [PATCH net-next v11] virtio_net: add page_pool support for buffer allocation

[Omar Elghoul <oelghoul@linux.ibm.com>]

On 3/23/26 11:52 AM, Michael S. Tsirkin wrote:
> On Mon, Mar 23, 2026 at 11:01:31AM -0400, Omar Elghoul wrote:
>> [...]
> Well... I am not sure how I missed it. Obvious in hindsight:
>
> static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>                          void *buf, unsigned int len, void **ctx,
>                          unsigned int *xdp_xmit,
>                          struct virtnet_rq_stats *stats)
> {
>          struct net_device *dev = vi->dev;
>          struct sk_buff *skb;
>          u8 flags;
>                  
>          if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
>                  pr_debug("%s: short packet %i\n", dev->name, len);
>                  DEV_STATS_INC(dev, rx_length_errors);
>                  virtnet_rq_free_buf(vi, rq, buf);
>                  return;
>          }
>          
>          /* About the flags below:
>           * 1. Save the flags early, as the XDP program might overwrite them.
>           * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
>           * stay valid after XDP processing.
>           * 2. XDP doesn't work with partially checksummed packets (refer to
>           * virtnet_xdp_set()), so packets marked as
>           * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
>           */
>                  
>          if (vi->mergeable_rx_bufs) {
>                  flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>                  skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
>                                          stats);
>          } else if (vi->big_packets) {
>                  void *p = page_address((struct page *)buf);
>                  
>                  flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
>                  skb = receive_big(dev, vi, rq, buf, len, stats);
>          } else {
>                  flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>                  skb = receive_small(dev, vi, rq, buf, ctx, len, xdp_xmit, stats);
>          }
>
>
> So we are reading the header, before dma sync, which is within
> receive_mergeable and friends:
Thank you for your analysis and explanation.
>
> static struct sk_buff *receive_mergeable(struct net_device *dev,
>                                           struct virtnet_info *vi,
>                                           struct receive_queue *rq,
>                                           void *buf,
>                                           void *ctx,
>                                           unsigned int len,
>                                           unsigned int *xdp_xmit,
>                                           struct virtnet_rq_stats *stats)
> {
>          struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
>          int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
>          struct page *page = virt_to_head_page(buf);
>          int offset = buf - page_address(page);
>          struct sk_buff *head_skb, *curr_skb;
>          unsigned int truesize = mergeable_ctx_to_truesize(ctx);
>          unsigned int headroom = mergeable_ctx_to_headroom(ctx);
>                  
>          head_skb = NULL;
>                  
>          if (rq->use_page_pool_dma)
>                  page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, len);
>                  
>
>
> Just as a test, the below should fix it (compiled only), but the real
> fix is more complex since we need to be careful to avoid expensive syncing
> twice.

I applied your patch and tested it on my system. With this change, I 
could not reproduce the same error anymore. I would be happy to test a 
proper fix once you have one.

>
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 97035b49bae7..57b4f5954bed 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -931,9 +931,19 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>   
>   static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx)
>   {
> +	void *buf;
> +
>   	BUG_ON(!rq->page_pool);
>   
> -	return virtqueue_get_buf_ctx(rq->vq, len, ctx);
> +	buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
> +	if (buf && rq->use_page_pool_dma && *len) {
> +		struct page *page = virt_to_head_page(buf);
> +		int offset = buf - page_address(page);
> +
> +		page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, *len);
> +	}
> +
> +	return buf;
>   }
>   
>   static void virtnet_rq_unmap_free_buf(struct virtqueue *vq, void *buf)
>
>
>
>