linux-kernel - [PATCH 5.12 437/677] xsk: Respect devices headroom and tailroom on generic xmit path

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Message-Id: <20210512144851.860267896@linuxfoundation.org>
Date:   Wed, 12 May 2021 16:48:03 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Alexander Lobakin <alobakin@...me>,
        Daniel Borkmann <daniel@...earbox.net>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.12 437/677] xsk: Respect devices headroom and tailroom on generic xmit path

From: Alexander Lobakin <alobakin@...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@...me>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
Acked-by: John Fastabend <john.fastabend@...il.com>
Link: https://lore.kernel.org/bpf/20210218204908.5455-5-alobakin@pm.me
Signed-off-by: Sasha Levin <sashal@...nel.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);
-- 
2.30.2