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Date:   Mon, 18 May 2020 17:37:04 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, brouer@...hat.com,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com
Subject: Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

On 5/18/20 12:00 PM, Toke Høiland-Jørgensen wrote:
> I meant 'less powerful' in the obvious sense: it only sees a subset of
> the packets going out of the interface. And so I worry that it will (a)
> make an already hard to use set of APIs even more confusing, and (b)
> turn out to not be enough so we'll end up needing a "real" egress hook.
> 
> As I said in my previous email, a post-REDIRECT hook may or may not be
> useful in its own right. I'm kinda on the fence about that, but am
> actually leaning towards it being useful; however, I am concerned that
> it'll end up being redundant if we do get a full egress hook.
> 

I made the changes to mlx5 to run programs in the driver back in early
March. I have looked at both i40e and mlx5 xmit functions all the way to
h/w handoff to get 2 vendor perspectives. With xdp I can push any header
I want - e.g., mpls - and as soon as I do the markers are wrong. Take a
look at mlx5e_sq_xmit and how it gets the transport header offset. Or
i40e_tso. Those markers are necessary for the offloads so there is no
'post skb' location to run a bpf program in the driver and have the
result be sane for hardware handoff.

[ as an aside, a co-worker just happened to hit something like this
today (unrelated to xdp). He called dev_queue_xmit with a large,
manually crafted packet and no skb markers. Both the boxes (connected
back to back) had to be rebooted.]

>From what I can see there are 3 ways to run an XDP program on skbs in
the Tx path:
1. disable hardware offloads (which is nonsense - you don't disable H/W
acceleration for S/W acceleration),

2. neuter XDP egress and not allow bpf_xdp_adjust_head (that is a key
feature of XDP), or

3. walk the skb afterwards and reset the markers (performance killer).

I have stared at this code for months; I would love for someone to prove
me wrong.

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