netdev - Re: [PATCH bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP programs in the egress path

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Date:   Mon, 27 Jan 2020 06:16:23 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        prashantbhole.linux@...il.com, jasowang@...hat.com,
        davem@...emloft.net, jbrouer@...hat.com, mst@...hat.com,
        toshiaki.makita1@...il.com, daniel@...earbox.net,
        john.fastabend@...il.com, ast@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP
 programs in the egress path

On Sun, 26 Jan 2020 21:03:15 -0700, David Ahern wrote:
> On 1/26/20 3:11 PM, Jakub Kicinski wrote:
> > I looked through the commit message and the cover letter again, and you
> > never explain why you need the egress hook. Could you please clarify
> > your needs?   
> 
> XDP is about efficient network processing - ie., bypassing the Linux
> stack when it does not make sense for the person deploying some
> solution. XDP right now is Rx centric.

Network hardware is also "Rx centric" and somehow it works..

> I want to run an ebpf program in the Tx path of the NIC regardless of
> how the packet arrived at the device -- as an skb or an xdp_frame. There
> are options for running programs on skb-based packets (e.g., tc). There
> are *zero* options for manipulating/controlling/denying xdp_frames -
> e.g., one REDIRECTED from an ingress device.

Okay - so no precise use case.  You can run the same program at the 
end of whatever is doing the redirect (especially with Alexei's work 
on linking) and from cls_bpf 🤷‍♂️

I'm sure all driver authors can't wait to plough through their TX paths
:/