From: Alexander Lobakin alobakin@pm.me
[ Upstream commit 3914d88f7608e6c2e80e344474fa289370c32451 ]
xsk_generic_xmit() allocates a new skb and then queues it for xmitting. The size of new skb's headroom is desc->len, so it comes to the driver/device with no reserved headroom and/or tailroom. Lots of drivers need some headroom (and sometimes tailroom) to prepend (and/or append) some headers or data, e.g. CPU tags, device-specific headers/descriptors (LSO, TLS etc.), and if case of no available space skb_cow_head() will reallocate the skb. Reallocations are unwanted on fast-path, especially when it comes to XDP, so generic XSK xmit should reserve the spaces declared in dev->needed_headroom and dev->needed tailroom to avoid them.
Note on max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom)):
Usually, output functions reserve LL_RESERVED_SPACE(dev), which consists of dev->hard_header_len + dev->needed_headroom, aligned by 16.
However, on XSK xmit hard header is already here in the chunk, so hard_header_len is not needed. But it'd still be better to align data up to cacheline, while reserving no less than driver requests for headroom. NET_SKB_PAD here is to double-insure there will be no reallocations even when the driver advertises no needed_headroom, but in fact need it (not so rare case).
Fixes: 35fcde7f8deb ("xsk: support for Tx") Signed-off-by: Alexander Lobakin alobakin@pm.me Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Magnus Karlsson magnus.karlsson@intel.com Acked-by: John Fastabend john.fastabend@gmail.com Link: https://lore.kernel.org/bpf/20210218204908.5455-5-alobakin@pm.me Signed-off-by: Sasha Levin sashal@kernel.org --- net/xdp/xsk.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c index 4faabd1ecfd1..143979ea4165 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c @@ -454,12 +454,16 @@ static int xsk_generic_xmit(struct sock *sk) struct sk_buff *skb; unsigned long flags; int err = 0; + u32 hr, tr;
mutex_lock(&xs->mutex);
if (xs->queue_id >= xs->dev->real_num_tx_queues) goto out;
+ hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom)); + tr = xs->dev->needed_tailroom; + while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) { char *buffer; u64 addr; @@ -471,11 +475,13 @@ static int xsk_generic_xmit(struct sock *sk) }
len = desc.len; - skb = sock_alloc_send_skb(sk, len, 1, &err); + skb = sock_alloc_send_skb(sk, hr + len + tr, 1, &err); if (unlikely(!skb)) goto out;
+ skb_reserve(skb, hr); skb_put(skb, len); + addr = desc.addr; buffer = xsk_buff_raw_get_data(xs->pool, addr); err = skb_store_bits(skb, 0, buffer, len);