netdev - Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

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Date:   Mon, 18 May 2020 20:00:45 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        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,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

David Ahern <dsahern@...il.com> writes:

> On 5/18/20 3:08 AM, Toke Høiland-Jørgensen wrote:
>> I can see your point that fixing up the whole skb after the program has
>> run is not a good idea. But to me that just indicates that the hook is
>> in the wrong place: that it really should be in the driver, executed at
>> a point where the skb data structure is no longer necessary (similar to
>> how the ingress hook is before the skb is generated).
>
> Have you created a cls_bpf program to modify skbs? Have you looked at
> the helpers, the restrictions and the tight management of skb changes?
> Have you followed the skb from create to device handoff through the
> drivers? Have you looked at the history of encapsulations, gso handling,
> offloads, ...?

Have you tried re-reading the first sentence of the paragraph you're
replying to? You know, the one that started with "I can see your point
that..."

>> Otherwise, what you're proposing is not an egress hook, but rather a
>> 'post-REDIRECT hook', which is strictly less powerful. This may or may
>> not be useful in its own right, but let's not pretend it's a full egress
>> hook. Personally I feel that the egress hook is what we should be going
>> for, not this partial thing.
>
> You are hand waving. Be specific, with details.

Are you deliberately trying to antagonise me or something? It's a really
odd way to try to make your case...

> Less powerful how? There are only so many operations you can do to a
> packet. What do you want to do and what can't be done with this proposed
> change? Why must it be done as XDP vs proper synergy between the 2 paths.

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.

-Toke