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

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Date:   Mon, 14 Mar 2022 23:43:05 +0100
From:   Felix Fietkau <nbd@....name>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "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 14.03.22 23:20, Daniel Borkmann wrote:
> 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?
I chose XDP because of bpf_redirect_map, which doesn't seem to be 
available to tc ingress classifier programs.

When I started writing the code, I didn't know that generic XDP 
performance would be bad on pretty much any ethernet/WLAN driver that 
wasn't updated to support it.

- Felix