Patch "xsk: Respect device's headroom and tailroom on generic xmit path" has been added to the 5.10-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 5.10-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     xsk-respect-device-s-headroom-and-tailroom-on-generi.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 4e75e4abbf0ecb277a2f69a7308e7f2918daf29f
Author: Alexander Lobakin <alobakin@xxxxx>
Date:   Thu Feb 18 20:50:31 2021 +0000

    xsk: Respect device's headroom and tailroom on generic xmit path
    
    [ 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@xxxxx>
    Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
    Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
    Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
    Link: https://lore.kernel.org/bpf/20210218204908.5455-5-alobakin@xxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 52fd1f96b241..ca4716b92774 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -380,12 +380,16 @@ 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;
@@ -397,11 +401,13 @@ 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);