netdev - Re: [PATCH bpf-next] bpf: add bpf_get_xdp

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Message-ID: <0333522d-7b65-e665-f19e-d36d11bd7846@iogearbox.net>
Date:   Mon, 31 Aug 2020 22:33:10 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Harshitha Ramamurthy <harshitha.ramamurthy@...el.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...nel.org,
        davem@...emloft.net, kuba@...nel.org
Cc:     dsahern@...il.com, alexander.h.duyck@...el.com,
        tom.herbert@...el.com
Subject: Re: [PATCH bpf-next] bpf: add bpf_get_xdp_hash helper function

On 8/31/20 9:25 PM, Harshitha Ramamurthy wrote:
> This patch adds a helper function called bpf_get_xdp_hash to calculate
> the hash for a packet at the XDP layer. In the helper function, we call
> the kernel flow dissector in non-skb mode by passing the net pointer
> to calculate the hash.

So this commit msg says 'what' the patch does, but says nothing about 'why' it is
needed especially given there's the 1 mio insn limit in place where it should be
easy to write that up in BPF anyway. The commit msg needs to have a clear rationale
which describes the motivation behind this helper.. why it cannot be done in BPF
itself?

> Changes since RFC:
> - accounted for vlans(David Ahern)
> - return the correct hash by not using skb_get_hash(David Ahern)
> - call __skb_flow_dissect in non-skb mode
>