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

[Jason Wang <jasowang@xxxxxxxxxx>]

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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