Linux Virtualization - Mar 2023 - [PATCH net-next 2/8] virtio_net: mergeable xdp: introduce mergeable_xdp_prepare

Separating the logic of preparation for xdp from receive_mergeable.

The purpose of this is to simplify the logic of execution of XDP.

The main logic here is that when headroom is insufficient, we need to
allocate a new page and calculate offset. It should be noted that if
there is new page, the variable page will refer to the new page.

Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
---
 drivers/net/virtio_net.c | 135 ++++++++++++++++++++++-----------------
 1 file changed, 77 insertions(+), 58 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4d2bf1ce0730..bb426958cdd4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1162,6 +1162,79 @@ static int virtnet_build_xdp_buff_mrg(struct net_device
*dev,
 	return 0;
 }
 
+static void *mergeable_xdp_prepare(struct virtnet_info *vi,
+				   struct receive_queue *rq,
+				   struct bpf_prog *xdp_prog,
+				   void *ctx,
+				   unsigned int *frame_sz,
+				   int *num_buf,
+				   struct page **page,
+				   int offset,
+				   unsigned int *len,
+				   struct virtio_net_hdr_mrg_rxbuf *hdr)
+{
+	unsigned int truesize = mergeable_ctx_to_truesize(ctx);
+	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
+	struct page *xdp_page;
+	unsigned int xdp_room;
+
+	/* Transient failure which in theory could occur if
+	 * in-flight packets from before XDP was enabled reach
+	 * the receive path after XDP is loaded.
+	 */
+	if (unlikely(hdr->hdr.gso_type))
+		return NULL;
+
+	/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
+	 * with headroom may add hole in truesize, which
+	 * make their length exceed PAGE_SIZE. So we disabled the
+	 * hole mechanism for xdp. See add_recvbuf_mergeable().
+	 */
+	*frame_sz = truesize;
+
+	/* This happens when headroom is not enough because
+	 * of the buffer was prefilled before XDP is set.
+	 * This should only happen for the first several packets.
+	 * In fact, vq reset can be used here to help us clean up
+	 * the prefilled buffers, but many existing devices do not
+	 * support it, and we don't want to bother users who are
+	 * using xdp normally.
+	 */
+	if (!xdp_prog->aux->xdp_has_frags &&
+	    (*num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
+		/* linearize data for XDP */
+		xdp_page = xdp_linearize_page(rq, num_buf,
+					      *page, offset,
+					      VIRTIO_XDP_HEADROOM,
+					      len);
+
+		if (!xdp_page)
+			return NULL;
+	} else if (unlikely(headroom < virtnet_get_headroom(vi))) {
+		xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
+					  sizeof(struct skb_shared_info));
+		if (*len + xdp_room > PAGE_SIZE)
+			return NULL;
+
+		xdp_page = alloc_page(GFP_ATOMIC);
+		if (!xdp_page)
+			return NULL;
+
+		memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
+		       page_address(*page) + offset, *len);
+	} else {
+		return page_address(*page) + offset;
+	}
+
+	*frame_sz = PAGE_SIZE;
+
+	put_page(*page);
+
+	*page = xdp_page;
+
+	return page_address(xdp_page) + VIRTIO_XDP_HEADROOM;
+}
+
 static struct sk_buff *receive_mergeable(struct net_device *dev,
 					 struct virtnet_info *vi,
 					 struct receive_queue *rq,
@@ -1181,7 +1254,7 @@ static struct sk_buff *receive_mergeable(struct net_device
*dev,
 	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
 	unsigned int tailroom = headroom ? sizeof(struct skb_shared_info) : 0;
 	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
-	unsigned int frame_sz, xdp_room;
+	unsigned int frame_sz;
 	int err;
 
 	head_skb = NULL;
@@ -1211,65 +1284,11 @@ static struct sk_buff *receive_mergeable(struct
net_device *dev,
 		u32 act;
 		int i;
 
-		/* Transient failure which in theory could occur if
-		 * in-flight packets from before XDP was enabled reach
-		 * the receive path after XDP is loaded.
-		 */
-		if (unlikely(hdr->hdr.gso_type))
+		data = mergeable_xdp_prepare(vi, rq, xdp_prog, ctx, &frame_sz,
&num_buf, &page,
+					     offset, &len, hdr);
+		if (!data)
 			goto err_xdp;
 
-		/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
-		 * with headroom may add hole in truesize, which
-		 * make their length exceed PAGE_SIZE. So we disabled the
-		 * hole mechanism for xdp. See add_recvbuf_mergeable().
-		 */
-		frame_sz = truesize;
-
-		/* This happens when headroom is not enough because
-		 * of the buffer was prefilled before XDP is set.
-		 * This should only happen for the first several packets.
-		 * In fact, vq reset can be used here to help us clean up
-		 * the prefilled buffers, but many existing devices do not
-		 * support it, and we don't want to bother users who are
-		 * using xdp normally.
-		 */
-		if (!xdp_prog->aux->xdp_has_frags &&
-		    (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
-			/* linearize data for XDP */
-			xdp_page = xdp_linearize_page(rq, &num_buf,
-						      page, offset,
-						      VIRTIO_XDP_HEADROOM,
-						      &len);
-			frame_sz = PAGE_SIZE;
-
-			if (!xdp_page)
-				goto err_xdp;
-			offset = VIRTIO_XDP_HEADROOM;
-
-			put_page(page);
-			page = xdp_page;
-		} else if (unlikely(headroom < virtnet_get_headroom(vi))) {
-			xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
-						  sizeof(struct skb_shared_info));
-			if (len + xdp_room > PAGE_SIZE)
-				goto err_xdp;
-
-			xdp_page = alloc_page(GFP_ATOMIC);
-			if (!xdp_page)
-				goto err_xdp;
-
-			memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
-			       page_address(page) + offset, len);
-			frame_sz = PAGE_SIZE;
-			offset = VIRTIO_XDP_HEADROOM;
-
-			put_page(page);
-			page = xdp_page;
-		} else {
-			xdp_page = page;
-		}
-
-		data = page_address(xdp_page) + offset;
 		err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, len, frame_sz,
 						 &num_buf, &xdp_frags_truesz, stats);
 		if (unlikely(err))
-- 
2.32.0.3.g01195cf9f

? 2023/3/28 20:04, Xuan Zhuo ??:
> Separating the logic of preparation for xdp from receive_mergeable.
>
> The purpose of this is to simplify the logic of execution of XDP.
>
> The main logic here is that when headroom is insufficient, we need to
> allocate a new page and calculate offset. It should be noted that if
> there is new page, the variable page will refer to the new page.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo at linux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 135 ++++++++++++++++++++++-----------------
>   1 file changed, 77 insertions(+), 58 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4d2bf1ce0730..bb426958cdd4 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1162,6 +1162,79 @@ static int virtnet_build_xdp_buff_mrg(struct
net_device *dev,
>   	return 0;
>   }
>   
> +static void *mergeable_xdp_prepare(struct virtnet_info *vi,
> +				   struct receive_queue *rq,
> +				   struct bpf_prog *xdp_prog,
> +				   void *ctx,
> +				   unsigned int *frame_sz,
> +				   int *num_buf,
> +				   struct page **page,
> +				   int offset,
> +				   unsigned int *len,
> +				   struct virtio_net_hdr_mrg_rxbuf *hdr)
> +{
> +	unsigned int truesize = mergeable_ctx_to_truesize(ctx);
> +	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
> +	struct page *xdp_page;
> +	unsigned int xdp_room;
> +
> +	/* Transient failure which in theory could occur if
> +	 * in-flight packets from before XDP was enabled reach
> +	 * the receive path after XDP is loaded.
> +	 */
> +	if (unlikely(hdr->hdr.gso_type))
> +		return NULL;
> +
> +	/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
> +	 * with headroom may add hole in truesize, which
> +	 * make their length exceed PAGE_SIZE. So we disabled the
> +	 * hole mechanism for xdp. See add_recvbuf_mergeable().
> +	 */
> +	*frame_sz = truesize;
> +
> +	/* This happens when headroom is not enough because
> +	 * of the buffer was prefilled before XDP is set.
> +	 * This should only happen for the first several packets.
> +	 * In fact, vq reset can be used here to help us clean up
> +	 * the prefilled buffers, but many existing devices do not
> +	 * support it, and we don't want to bother users who are
> +	 * using xdp normally.
> +	 */
> +	if (!xdp_prog->aux->xdp_has_frags &&
> +	    (*num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
> +		/* linearize data for XDP */
> +		xdp_page = xdp_linearize_page(rq, num_buf,
> +					      *page, offset,
> +					      VIRTIO_XDP_HEADROOM,
> +					      len);
> +
> +		if (!xdp_page)
> +			return NULL;
> +	} else if (unlikely(headroom < virtnet_get_headroom(vi))) {
> +		xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
> +					  sizeof(struct skb_shared_info));
> +		if (*len + xdp_room > PAGE_SIZE)
> +			return NULL;
> +
> +		xdp_page = alloc_page(GFP_ATOMIC);
> +		if (!xdp_page)
> +			return NULL;
> +
> +		memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
> +		       page_address(*page) + offset, *len);
> +	} else {
> +		return page_address(*page) + offset;

This makes the code a little harder to be read than the original code.

Why not do a verbatim moving without introducing new logic? (Or 
introducing new logic on top?)

Thanks

> +	}
> +
> +	*frame_sz = PAGE_SIZE;
> +
> +	put_page(*page);
> +
> +	*page = xdp_page;
> +
> +	return page_address(xdp_page) + VIRTIO_XDP_HEADROOM;
> +}
> +
>   static struct sk_buff *receive_mergeable(struct net_device *dev,
>   					 struct virtnet_info *vi,
>   					 struct receive_queue *rq,
> @@ -1181,7 +1254,7 @@ static struct sk_buff *receive_mergeable(struct
net_device *dev,
>   	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
>   	unsigned int tailroom = headroom ? sizeof(struct skb_shared_info) : 0;
>   	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
> -	unsigned int frame_sz, xdp_room;
> +	unsigned int frame_sz;
>   	int err;
>   
>   	head_skb = NULL;
> @@ -1211,65 +1284,11 @@ static struct sk_buff *receive_mergeable(struct
net_device *dev,
>   		u32 act;
>   		int i;
>   
> -		/* Transient failure which in theory could occur if
> -		 * in-flight packets from before XDP was enabled reach
> -		 * the receive path after XDP is loaded.
> -		 */
> -		if (unlikely(hdr->hdr.gso_type))
> +		data = mergeable_xdp_prepare(vi, rq, xdp_prog, ctx, &frame_sz,
&num_buf, &page,
> +					     offset, &len, hdr);
> +		if (!data)
>   			goto err_xdp;
>   
> -		/* Now XDP core assumes frag size is PAGE_SIZE, but buffers
> -		 * with headroom may add hole in truesize, which
> -		 * make their length exceed PAGE_SIZE. So we disabled the
> -		 * hole mechanism for xdp. See add_recvbuf_mergeable().
> -		 */
> -		frame_sz = truesize;
> -
> -		/* This happens when headroom is not enough because
> -		 * of the buffer was prefilled before XDP is set.
> -		 * This should only happen for the first several packets.
> -		 * In fact, vq reset can be used here to help us clean up
> -		 * the prefilled buffers, but many existing devices do not
> -		 * support it, and we don't want to bother users who are
> -		 * using xdp normally.
> -		 */
> -		if (!xdp_prog->aux->xdp_has_frags &&
> -		    (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
> -			/* linearize data for XDP */
> -			xdp_page = xdp_linearize_page(rq, &num_buf,
> -						      page, offset,
> -						      VIRTIO_XDP_HEADROOM,
> -						      &len);
> -			frame_sz = PAGE_SIZE;
> -
> -			if (!xdp_page)
> -				goto err_xdp;
> -			offset = VIRTIO_XDP_HEADROOM;
> -
> -			put_page(page);
> -			page = xdp_page;
> -		} else if (unlikely(headroom < virtnet_get_headroom(vi))) {
> -			xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
> -						  sizeof(struct skb_shared_info));
> -			if (len + xdp_room > PAGE_SIZE)
> -				goto err_xdp;
> -
> -			xdp_page = alloc_page(GFP_ATOMIC);
> -			if (!xdp_page)
> -				goto err_xdp;
> -
> -			memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
> -			       page_address(page) + offset, len);
> -			frame_sz = PAGE_SIZE;
> -			offset = VIRTIO_XDP_HEADROOM;
> -
> -			put_page(page);
> -			page = xdp_page;
> -		} else {
> -			xdp_page = page;
> -		}
> -
> -		data = page_address(xdp_page) + offset;
>   		err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, len,
frame_sz,
>   						 &num_buf, &xdp_frags_truesz, stats);
>   		if (unlikely(err))