netdev - Re: [PATCH RFC v4 bpf-next 03/11] xdp: Add xdp_txq_info to xdp

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Message-ID: <87lfonuind.fsf@toke.dk>
Date:   Fri, 28 Feb 2020 11:10:30 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     David Ahern <dahern@...italocean.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, jasowang@...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,
        dsahern@...il.com
Subject: Re: [PATCH RFC v4 bpf-next 03/11] xdp: Add xdp_txq_info to xdp_buff

David Ahern <dahern@...italocean.com> writes:

> On 2/27/20 4:58 AM, Toke Høiland-Jørgensen wrote:
>>  also, an egress program may want to actually know which
>> ingress iface the packet was first received on. So why not just keep
>> both fields? Since ifindex 0 is invalid anyway, the field could just be
>> 0 when it isn't known (e.g., egress ifindex on RX, or ingress ifindex if
>> it comes from the stack)?
>
> Today, the ingress device is lost in the conversion from xdp_buff to
> xdp_frame. The plumbing needed to keep that information is beyond the
> scope of this set.
>
> I am open to making the UAPI separate entries if there is a real reason
> for it. Do you have a specific use case?

I was thinking it could be a nice shorthand for whether a packet comes
from the local stack or was forwarded (0 == stack). But no, I don't have
a concrete application where this is useful. However, if we define it as
a union we lose the ability to change our mind. Together with the
debugability issue I just replied with to your other email, I think it
would be better to expend the four bytes keep them as separate fields,
but still restrict access to the RX ifindex for now.

-Toke