From 9ce6146ec7b50718fa5ef5287f1d6561b25a5da8 Mon Sep 17 00:00:00 2001 From: Jesper Dangaard Brouer Date: Thu, 14 May 2020 12:50:44 +0200 Subject: [PATCH] virtio_net: Add XDP frame size in two code paths The virtio_net driver is running inside the guest-OS. There are two XDP receive code-paths in virtio_net, namely receive_small() and receive_mergeable(). The receive_big() function does not support XDP. In receive_small() the frame size is available in buflen. The buffer backing these frames are allocated in add_recvbuf_small() with same size, except for the headroom, but tailroom have reserved room for skb_shared_info. The headroom is encoded in ctx pointer as a value. In receive_mergeable() the frame size is more dynamic. There are two basic cases: (1) buffer size is based on a exponentially weighted moving average (see DECLARE_EWMA) of packet length. Or (2) in case virtnet_get_headroom() have any headroom then buffer size is PAGE_SIZE. The ctx pointer is this time used for encoding two values; the buffer len "truesize" and headroom. In case (1) if the rx buffer size is underestimated, the packet will have been split over more buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of buffer area). If that happens the XDP path does a xdp_linearize_page operation. V3: Adjust frame_sz in receive_mergeable() case, spotted by Jason Wang. The code is really hard to follow, so some hints to reviewers. The receive_mergeable() case gets frames that were allocated in add_recvbuf_mergeable() which uses headroom=virtnet_get_headroom(), and 'buf' ptr is advanced this headroom. The headroom can only be 0 or VIRTIO_XDP_HEADROOM, as virtnet_get_headroom is really simple: static unsigned int virtnet_get_headroom(struct virtnet_info *vi) { return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0; } As frame_sz is an offset size from xdp.data_hard_start, reviewers should notice how this is calculated in receive_mergeable(): int offset = buf - page_address(page); [...] data = page_address(xdp_page) + offset; xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len; The calculated offset will always be VIRTIO_XDP_HEADROOM when reaching this code. Thus, xdp.data_hard_start will be page-start address plus vi->hdr_len. Given this xdp.frame_sz need to be reduced with vi->hdr_len size. IMHO a followup patch should cleanup this code to make it easier to maintain and understand, but it is outside the scope of this patchset. Signed-off-by: Jesper Dangaard Brouer Signed-off-by: Alexei Starovoitov Acked-by: Michael S. Tsirkin Acked-by: Jason Wang Link: https://lore.kernel.org/bpf/158945344436.97035.9445115070189151680.stgit@firesoul --- drivers/net/virtio_net.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 11f722460513..9e1b5d748586 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -689,6 +689,7 @@ static struct sk_buff *receive_small(struct net_device *dev, xdp.data_end = xdp.data + len; xdp.data_meta = xdp.data; xdp.rxq = &rq->xdp_rxq; + xdp.frame_sz = buflen; orig_data = xdp.data; act = bpf_prog_run_xdp(xdp_prog, &xdp); stats->xdp_packets++; @@ -797,10 +798,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev, int offset = buf - page_address(page); struct sk_buff *head_skb, *curr_skb; struct bpf_prog *xdp_prog; - unsigned int truesize; + unsigned int truesize = mergeable_ctx_to_truesize(ctx); unsigned int headroom = mergeable_ctx_to_headroom(ctx); - int err; unsigned int metasize = 0; + unsigned int frame_sz; + int err; head_skb = NULL; stats->bytes += len - vi->hdr_len; @@ -821,6 +823,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev, if (unlikely(hdr->hdr.gso_type)) goto err_xdp; + /* Buffers with headroom use PAGE_SIZE as alloc size, + * see add_recvbuf_mergeable() + get_mergeable_buf_len() + */ + frame_sz = headroom ? PAGE_SIZE : truesize; + /* This happens when rx buffer size is underestimated * or headroom is not enough because of the buffer * was refilled before XDP is set. This should only @@ -834,6 +841,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev, page, offset, VIRTIO_XDP_HEADROOM, &len); + frame_sz = PAGE_SIZE; + if (!xdp_page) goto err_xdp; offset = VIRTIO_XDP_HEADROOM; @@ -850,6 +859,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev, xdp.data_end = xdp.data + (len - vi->hdr_len); xdp.data_meta = xdp.data; xdp.rxq = &rq->xdp_rxq; + xdp.frame_sz = frame_sz - vi->hdr_len; act = bpf_prog_run_xdp(xdp_prog, &xdp); stats->xdp_packets++; @@ -924,7 +934,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev, } rcu_read_unlock(); - 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); -- GitLab