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

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Date:   Sat, 01 Feb 2020 17:03:19 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org, prashantbhole.linux@...il.com,
        jasowang@...hat.com, davem@...emloft.net, 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 Tue, 28 Jan 2020 15:13:43 +0100, Jesper Dangaard Brouer wrote:
>> On Sun, 26 Jan 2020 14:17:01 -0800
>> Jakub Kicinski <kuba@...nel.org> wrote:
>> 
>> > On Sun, 26 Jan 2020 13:49:33 +0100, Jesper Dangaard Brouer wrote:  
>> > > Yes, please. I want this NIC TX hook to see both SKBs and xdp_frames.    
>> > 
>> > Any pointers on what for? Unless we see actual use cases there's
>> > a justifiable concern of the entire thing just being an application of
>> > "We can solve any problem by introducing an extra level of indirection."  
>> 
>> I have two use-cases:
>> 
>> (1) For XDP easier handling of interface specific setting on egress,
>> e.g. pushing a VLAN-id, instead of having to figure this out in RX hook.
>> (I think this is also David Ahern's use-case)
>
> Is it really useful to have a hook before multi-buffer frames are
> possible and perhaps TSO? The local TCP performance is going to tank
> with XDP enabled otherwise.

For a software router (i.e., something that mostly forwards packets) it
can still be useful without multi-buffer. But yeah, that is definitely
something we need to solve, regardless of where this goes.

>> (2) I want this egress XDP hook to have the ability to signal
>> backpressure. Today we have BQL in most drivers (which is essential to
>> avoid bufferbloat). For XDP_REDIRECT we don't, which we must solve.
>> 
>> For use-case(2), we likely need a BPF-helper calling netif_tx_stop_queue(),
>> or a return code that can stop the queue towards the higher layers.
>
> Agreed, although for that use case, I'm not sure if non-XDP frames 
> have to pass trough the hook. Hard to tell as the current patches 
> don't touch on this use case.

I think it would be weird and surprising if it *doesn't* see packets
from the stack. On RX, XDP sees everything; the natural expectation
would be that this was also the case on TX, no?

-Toke