xsk: Respect device's headroom and tailroom on generic xmit path

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: 35fcde7f ("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

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);