netdev - Re: [PATCH net] xsk: respect the offsets when copying frags

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Message-ID: <aAkrFcSpxwdb+2tA@boxer>
Date: Wed, 23 Apr 2025 20:01:57 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Bui Quang Minh <minhquangbui99@...il.com>
CC: Stanislav Fomichev <stfomichev@...il.com>, <netdev@...r.kernel.org>,
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
	<pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Alexander Lobakin
	<aleksander.lobakin@...el.com>, <bpf@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] xsk: respect the offsets when copying frags

On Wed, Apr 23, 2025 at 09:58:16PM +0700, Bui Quang Minh wrote:
> On 4/23/25 21:41, Stanislav Fomichev wrote:
> > On 04/23, Bui Quang Minh wrote:
> > > Add the missing offsets when copying frags in xdp_copy_frags_from_zc().
> > Can you please share more about how you've hit this problem?
> > I don't see the caller of this function (xdp_build_skb_from_zc)
> > being used at all.
> > 
> > Alexander, do you have plans to use it? Or should we remove it for now?

libeth xdp support uses this:
https://lore.kernel.org/netdev/20250415172825.3731091-15-aleksander.lobakin@intel.com/

> Hi,
> 
> I've been playing around to add support for zerocopy XDP socket with multi
> buffer mergeable buffer in virtio-net (this TODO: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/drivers/net/virtio_net.c#n1312).
> In that case, I'll have a XDP buff with frags. When we have XDP_PASS return,
> I need to convert the XDP buff with frags to skb with frags, so I think the
> helper is quite helpful. I used it and got packet dropped due to checksum
> error. Debugging the problem, I've found out this issue which makes the
> skb's frag data incorrect.

Nice! I'll take a look at patch content tomorrow and probably ack it.

> 
> Thanks,
> Quang Minh.