On 2020/4/23 上午12:09, Jesper Dangaard Brouer wrote:
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.
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
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..1df3676da185 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;
Should this be PAGE_SIZE - SKB_DATA_ALIGN(sizeof(struct skb_shared_info))?
+
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;
Maybe we can easily check by
xdp.frame_sz = (xdp_page == page) ? truesize : ...
Thanks
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);
