Re: [PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add_*()

Michael S. Tsirkin
2014-09-07T07:20:32Z

On Fri, Sep 05, 2014 at 12:40:50PM +0200, Paolo Bonzini wrote:
> Il 03/09/2014 06:29, Rusty Russell ha scritto:
> > + sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
>
> I think 2 is enough here. That said...
>
> > sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
> > -
> > skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
> >
> > err = virtqueue_add_inbuf(rq->vq, rq->sg, 2, skb, gfp);
>
> ... skb_to_sgvec will already make the sg well formed, so the
> sg_init_table is _almost_ redundant; it is only there to remove
> intermediate end marks. The block layer takes care to remove
> them, but skb_to_sgvec doesn't.
>
> If the following patch can be accepted to net/core/skbuff.c, the
> sg_init_table in virtnet_alloc_queues will suffice.
>
> Paolo

You will have to post it to netdev as a new topic and Cc
Dave Miller for it to be considered.

> -------------------- 8< -------------------
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH] net: skb_to_sgvec: do not leave intermediate marks in the sgvec
>
> sg_set_buf/sg_set_page will leave the end mark in place in their
> argument, which may be in the middle of a scatterlist. If we
> remove the mark before calling them, we can avoid calls to
> sg_init_table before skb_to_sgvec.
>
> However, users of skb_to_sgvec_nomark now need to be careful and
> possibly restore the mark.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 163b673f9e62..a3108ef1f1c0 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3265,6 +3265,7 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
> if (copy > 0) {
> if (copy > len)
> copy = len;
> + sg_unmark_end(sg);
> sg_set_buf(sg, skb->data + offset, copy);
> elt++;
> if ((len -= copy) == 0)
> @@ -3283,6 +3284,7 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
>
> if (copy > len)
> copy = len;
> + sg_unmark_end(&sg[elt]);
> sg_set_page(&sg[elt], skb_frag_page(frag), copy,
> frag->page_offset+offset-start);
> elt++;
> @@ -3322,7 +3324,7 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
> * Scenario to use skb_to_sgvec_nomark:
> * 1. sg_init_table
> * 2. skb_to_sgvec_nomark(payload1)
> - * 3. skb_to_sgvec_nomark(payload2)
> + * 3. skb_to_sgvec(payload2)
> *
> * This is equivalent to:
> * 1. sg_init_table
> diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
> index a2afa89513a0..9ae5756d9e5f 100644
> --- a/net/ipv4/ah4.c
> +++ b/net/ipv4/ah4.c
> @@ -227,6 +227,7 @@ static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
> *seqhi = htonl(XFRM_SKB_CB(skb)->seq.output.hi);
> sg_set_buf(seqhisg, seqhi, seqhi_len);
> }
> + sg_mark_end(&sg[nfrags + sglists]);
> ahash_request_set_crypt(req, sg, icv, skb->len + seqhi_len);
> ahash_request_set_callback(req, 0, ah_output_done, skb);
>
> @@ -395,6 +396,7 @@ static int ah_input(struct xfrm_state *x, struct sk_buff *skb)
> *seqhi = XFRM_SKB_CB(skb)->seq.input.hi;
> sg_set_buf(seqhisg, seqhi, seqhi_len);
> }
> + sg_mark_end(&sg[nfrags + sglists]);
> ahash_request_set_crypt(req, sg, icv, skb->len + seqhi_len);
> ahash_request_set_callback(req, 0, ah_input_done, skb);
>
> diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
> index 72a4930bdc0a..c680d82e43de 100644
> --- a/net/ipv6/ah6.c
> +++ b/net/ipv6/ah6.c
> @@ -430,6 +430,8 @@ static int ah6_output(struct xfrm_state *x, struct sk_buff *skb)
> *seqhi = htonl(XFRM_SKB_CB(skb)->seq.output.hi);
> sg_set_buf(seqhisg, seqhi, seqhi_len);
> }
> + sg_mark_end(&sg[nfrags + sglists]);
> +
> ahash_request_set_crypt(req, sg, icv, skb->len + seqhi_len);
> ahash_request_set_callback(req, 0, ah6_output_done, skb);
>
> @@ -608,6 +610,7 @@ static int ah6_input(struct xfrm_state *x, struct sk_buff *skb)
> *seqhi = XFRM_SKB_CB(skb)->seq.input.hi;
> sg_set_buf(seqhisg, seqhi, seqhi_len);
> }
> + sg_mark_end(&sg[nfrags + sglists]);
>
> ahash_request_set_crypt(req, sg, icv, skb->len + seqhi_len);
> ahash_request_set_callback(req, 0, ah6_input_done, skb);