netdev - Re: [PATCH] net: xdp: allow user space to request a smaller packet headroom requirement

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <a61aef96-5364-e5a5-3827-e84da0c11218@iogearbox.net>
Date:   Mon, 14 Mar 2022 23:20:05 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Felix Fietkau <nbd@....name>,
        "Jesper D. Brouer" <netdev@...uer.com>, netdev@...r.kernel.org,
        bpf <bpf@...r.kernel.org>
Cc:     brouer@...hat.com, John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH] net: xdp: allow user space to request a smaller packet
 headroom requirement

On 3/14/22 11:16 PM, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd@....name> writes:
>> On 14.03.22 21:39, Jesper D. Brouer wrote:
>>> (Cc. BPF list and other XDP maintainers)
>>> On 14/03/2022 11.22, Felix Fietkau wrote:
>>>> Most ethernet drivers allocate a packet headroom of NET_SKB_PAD. Since it is
>>>> rounded up to L1 cache size, it ends up being at least 64 bytes on the most
>>>> common platforms.
>>>> On most ethernet drivers, having a guaranteed headroom of 256 bytes for XDP
>>>> adds an extra forced pskb_expand_head call when enabling SKB XDP, which can
>>>> be quite expensive.
>>>> Many XDP programs need only very little headroom, so it can be beneficial
>>>> to have a way to opt-out of the 256 bytes headroom requirement.
>>>
>>> IMHO 64 bytes is too small.
>>> We are using this area for struct xdp_frame and also for metadata
>>> (XDP-hints).  This will limit us from growing this structures for
>>> the sake of generic-XDP.
>>>
>>> I'm fine with reducting this to 192 bytes, as most Intel drivers
>>> have this headroom, and have defacto established that this is
>>> a valid XDP headroom, even for native-XDP.
>>>
>>> We could go a small as two cachelines 128 bytes, as if xdp_frame
>>> and metadata grows above a cache-line (64 bytes) each, then we have
>>> done something wrong (performance wise).
>> Here's some background on why I chose 64 bytes: I'm currently
>> implementing a userspace + xdp program to act as generic fastpath to
>> speed network bridging.
> 
> Any reason this can't run in the TC ingress hook instead? Generic XDP is
> a bit of an odd duck, and I'm not a huge fan of special-casing it this
> way...

+1, would have been fine with generic reduction to just down to 192 bytes
(though not less than that), but 64 is a bit too little. Also curious on
why not tc ingress instead?

Thanks,
Daniel