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

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Date:   Wed, 2 Sep 2020 00:54:38 +0000
From:   "Ramamurthy, Harshitha" <harshitha.ramamurthy@...el.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
CC:     "dsahern@...il.com" <dsahern@...il.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        "Herbert, Tom" <tom.herbert@...el.com>
Subject: RE: [PATCH bpf-next] bpf: add bpf_get_xdp_hash helper function

> From: Daniel Borkmann <daniel@...earbox.net>
> Sent: Monday, August 31, 2020 1:33 PM
> To: Ramamurthy, Harshitha <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; Duyck, Alexander H
> <alexander.h.duyck@...el.com>; Herbert, Tom <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?

Okay, will add a rationale in the commit message in the next version for use-case. 

Thanks,
Harshitha
> 
> > 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
> >