linux-kernel - Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE

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Date:   Tue, 18 Apr 2023 13:12:00 +0200
From:   Kal Cutter Conley <kal.conley@...tris.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/3] xsk: Support UMEM chunk_size > PAGE_SIZE

> >> In addition, presumably when using this mode, the other XDP actions
> >> (XDP_PASS, XDP_REDIRECT to other targets) would stop working unless we
> >> add special handling for that in the kernel? We'll definitely need to
> >> handle that somehow...
> >
> > I am not familiar with all the details here. Do you know a reason why
> > these cases would stop working / why special handling would be needed?
> > For example, if I have a UMEM that uses hugepages and XDP_PASS is
> > returned, then the data is just copied into an SKB right? SKBs can
> > also be created directly from hugepages AFAIK. So I don't understand
> > what the issue would be. Can someone explain this concern?
>
> Well, I was asking :) It may well be that the SKB path just works; did
> you test this? Pretty sure XDP_REDIRECT to another device won't, though?
>

I was also asking :-)

I tested that the SKB path is usable today with this patch.
Specifically, sending and receiving large jumbo packets with AF_XDP
and that a non-multi-buffer XDP program could access the whole packet.
I have not specifically tested XDP_REDIRECT to another device or
anything with ZC since that is not possible without driver support.

My feeling is, there wouldn't be non-trivial issues here since this
patchset changes nothing except allowing the maximum chunk size to be
larger. The driver either supports larger MTUs with XDP enabled or it
doesn't. If it doesn't, the frames are dropped anyway. Also, chunk
size mismatches between two XSKs (e.g. with XDP_REDIRECT) would be
something supported or not supported irrespective of this patchset.