linux-kernel - Re: [PATCH net-next] xdp: add xdp_skb_reserve

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Message-ID: <681274309ee3_30bc5e29490@willemb.c.googlers.com.notmuch>
Date: Wed, 30 Apr 2025 15:04:16 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jon Kohler <jon@...anix.com>, 
 Daniel Borkmann <daniel@...earbox.net>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, 
 Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Alexei Starovoitov <ast@...nel.org>, 
 Jesper Dangaard Brouer <hawk@...nel.org>, 
 John Fastabend <john.fastabend@...il.com>, 
 Simon Horman <horms@...nel.org>, 
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
 "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next] xdp: add xdp_skb_reserve_put helper

Jon Kohler wrote:
> 
> 
> > On Apr 30, 2025, at 2:40 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> > 
> > !-------------------------------------------------------------------|
> > CAUTION: External Email
> > 
> > |-------------------------------------------------------------------!
> > 
> > On 4/30/25 8:25 PM, Willem de Bruijn wrote:
> >> Jon Kohler wrote:
> >>> Add helper for calling skb_{put|reserve} to reduce repetitive pattern
> >>> across various drivers.
> >>> 
> >>> Plumb into tap and tun to start.
> >>> 
> >>> No functional change intended.
> >>> 
> >>> Signed-off-by: Jon Kohler <jon@...anix.com>
> >>> ---
> >>>  drivers/net/tap.c | 3 +--
> >>>  drivers/net/tun.c | 3 +--
> >>>  include/net/xdp.h | 8 ++++++++
> >>>  net/core/xdp.c    | 3 +--
> >>>  4 files changed, 11 insertions(+), 6 deletions(-)
> >> Subjective, but I prefer the existing code. I understand what
> >> skb_reserve and skb_put do. While xdp_skb_reserve_put adds a layer of
> >> indirection that I'd have to follow.
> >> Sometimes deduplication makes sense, sometimes the indirection adds
> >> more mental load than it's worth. In this case the code savings are
> >> small. As said, subjective. Happy to hear other opinions.
> > 
> > +1, agree with Willem
> 
> That’s a fair point. I was also toying with the idea of something like
> this instead:
> 
> e.g.
> xdp_headroom(xdp) == xdp->data - xdp->data_hard_start
> … similar to skb_headroom
> 
> xdp_length_base(xdp) == xdp->data_end - xdp->data
> … similar to xdp_get_buff_len, but doesn’t look at frags
> 
> then we could do:
> skb_reserve(skb, xdp_headroom(xdp));
> skb_put(skb, xdp_length_base(xdp));
> 
> Names TBD of course, but thoughts?
> 
> That way we keep skb_reserve/put just the same, but have
> a nice helper like we do for skb_headroom() already

I like the idea of xdp_headroom and xdk_headlen, similar to
skb_headroom and skb_headlen.