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

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Message-ID: <87o8usg92d.fsf@toke.dk>
Date:   Fri, 24 Jan 2020 16:36:58 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...il.com>
Cc:     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

Jakub Kicinski <kuba@...nel.org> writes:

> On Thu, 23 Jan 2020 14:33:42 -0700, David Ahern wrote:
>> On 1/23/20 4:35 AM, Toke Høiland-Jørgensen wrote:
>> > David Ahern <dsahern@...nel.org> writes:
>> >> From: David Ahern <dahern@...italocean.com>
>> >>
>> >> Add IFLA_XDP_EGRESS to if_link.h uapi to handle an XDP program attached
>> >> to the egress path of a device. Add rtnl_xdp_egress_fill and helpers as
>> >> the egress counterpart to the existing rtnl_xdp_fill. The expectation
>> >> is that going forward egress path will acquire the various levels of
>> >> attach - generic, driver and hardware.  
>> > 
>> > How would a 'hardware' attach work for this? As I said in my reply to
>> > the previous patch, isn't this explicitly for emulating XDP on the other
>> > end of a point-to-point link? How would that work with offloaded
>> > programs?
>> 
>> Nothing about this patch set is limited to point-to-point links.
>
> I struggle to understand of what the expected semantics of this new
> hook are. Is this going to be run on all frames sent to the device
> from the stack? All frames from the stack and from XDP_REDIRECT?
>
> A little hard to figure out the semantics when we start from a funky
> device like tun :S

Yes, that is also why I found this a bit weird. We have discussed plans
for an XDP TX hook before:
https://github.com/xdp-project/xdp-project/blob/master/xdp-project.org#xdp-hook-at-tx

That TX hook would run for everything at TX, but it would be a separate
program type with its own metadata access. Whereas the idea with this
series (seemed to me) to be just to be able to "emulate" run a regular
RX-side XDP program on egress for devices where this makes sense.

If this series is not meant to implement that "emulation", but rather be
usable for all devices, I really think we should go straight for the
full TX hook as discussed earlier...

-Toke