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Date:   Tue, 16 Feb 2021 14:34:50 +0000
To:     Magnus Karlsson <magnus.karlsson@intel.com>,
        =?utf-8?Q?Bj=C3=B6rn_T=C3=B6pel?= <bjorn@kernel.org>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Dust Li <dust.li@linux.alibaba.com>,
        Alexander Lobakin <alobakin@pm.me>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: [PATCH v5 bpf-next 5/6] xsk: respect device's headroom and tailroom on generic xmit path
Message-ID: <20210216143333.5861-6-alobakin@pm.me>
In-Reply-To: <20210216143333.5861-1-alobakin@pm.me>
References: <20210216143333.5861-1-alobakin@pm.me>
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Precedence: bulk

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>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
---
 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)
 =09struct sk_buff *skb;
 =09unsigned long flags;
 =09int err =3D 0;
+=09u32 hr, tr;
=20
 =09mutex_lock(&xs->mutex);
=20
 =09if (xs->queue_id >=3D xs->dev->real_num_tx_queues)
 =09=09goto out;
=20
+=09hr =3D max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom));
+=09tr =3D xs->dev->needed_tailroom;
+
 =09while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
 =09=09char *buffer;
 =09=09u64 addr;
@@ -471,11 +475,13 @@ static int xsk_generic_xmit(struct sock *sk)
 =09=09}
=20
 =09=09len =3D desc.len;
-=09=09skb =3D sock_alloc_send_skb(sk, len, 1, &err);
+=09=09skb =3D sock_alloc_send_skb(sk, hr + len + tr, 1, &err);
 =09=09if (unlikely(!skb))
 =09=09=09goto out;
=20
+=09=09skb_reserve(skb, hr);
 =09=09skb_put(skb, len);
+
 =09=09addr =3D desc.addr;
 =09=09buffer =3D xsk_buff_raw_get_data(xs->pool, addr);
 =09=09err =3D skb_store_bits(skb, 0, buffer, len);
--=20
2.30.1