Re: [PATCH net-next 2/5] virtio-net: pack headroom into ctx for mergeable buffer

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On 2017年07月19日 02:59, Michael S. Tsirkin wrote:
On Mon, Jul 17, 2017 at 08:43:58PM +0800, Jason Wang wrote:
Pack headroom into ctx, then during XDP set, we could know the size of
headroom and copy if needed. This is required for avoiding reset on
XDP.
Not really when XDP is set - it's when buffers are used.

Of course :)


virtio-net: pack headroom into ctx for mergeable buffers

Pack headroom into ctx - this way when we get a buffer we can figure out
the actual headroom that was allocated for the buffer. Will be helpful
to optimize switching between XDP and non-XDP modes which have different
headroom requirements.

Thanks, let me use this as the commit log.


Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++-----
  1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 1f8c15c..8fae9a8 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -270,6 +270,23 @@ static void skb_xmit_done(struct virtqueue *vq)
  		netif_wake_subqueue(vi->dev, vq2txq(vq));
  }
+#define MRG_CTX_HEADER_SHIFT 22
+static void *mergeable_len_to_ctx(unsigned int truesize,
+				  unsigned int headroom)
+{
+	return (void *)(unsigned long)((headroom << MRG_CTX_HEADER_SHIFT) | truesize);
+}
+
+static unsigned int mergeable_ctx_to_headroom(void *mrg_ctx)
+{
+	return (unsigned long)mrg_ctx >> MRG_CTX_HEADER_SHIFT;
+}
+
+static unsigned int mergeable_ctx_to_truesize(void *mrg_ctx)
+{
+	return (unsigned long)mrg_ctx & ((1 << MRG_CTX_HEADER_SHIFT) - 1);
+}
+
  /* Called from bottom half context */
  static struct sk_buff *page_to_skb(struct virtnet_info *vi,
  				   struct receive_queue *rq,
@@ -639,13 +656,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  	}
  	rcu_read_unlock();
- if (unlikely(len > (unsigned long)ctx)) {
+	truesize = mergeable_ctx_to_truesize(ctx);
+	if (unlikely(len > truesize)) {
  		pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
  			 dev->name, len, (unsigned long)ctx);
  		dev->stats.rx_length_errors++;
  		goto err_skb;
  	}
-	truesize = (unsigned long)ctx;
+
  	head_skb = page_to_skb(vi, rq, page, offset, len, truesize);
  	curr_skb = head_skb;
@@ -665,13 +683,14 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
  		}
page = virt_to_head_page(buf);
-		if (unlikely(len > (unsigned long)ctx)) {
+
+		truesize = mergeable_ctx_to_truesize(ctx);
+		if (unlikely(len > truesize)) {
  			pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
  				 dev->name, len, (unsigned long)ctx);
  			dev->stats.rx_length_errors++;
  			goto err_skb;
  		}
-		truesize = (unsigned long)ctx;
num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
  		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
@@ -889,7 +908,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
  	buf += headroom; /* advance address leaving hole at front of pkt */
-	ctx = (void *)(unsigned long)len;
+	ctx = mergeable_len_to_ctx(len, headroom);
  	get_page(alloc_frag->page);
  	alloc_frag->offset += len + headroom;
  	hole = alloc_frag->size - alloc_frag->offset;
--
2.7.4

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