netdev - Re: [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and add new AF

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Date:   Wed, 20 Jan 2021 16:18:50 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com
Subject: Re: [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and
 add new AF_XDP BPF helper

On 2021-01-20 15:54, Toke Høiland-Jørgensen wrote:
> Björn Töpel <bjorn.topel@...el.com> writes:
> 
>> On 2021-01-20 13:50, Toke Høiland-Jørgensen wrote:
>>> Björn Töpel <bjorn.topel@...il.com> writes:
>>>
>>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>>> index c001766adcbc..bbc7d9a57262 100644
>>>> --- a/include/uapi/linux/bpf.h
>>>> +++ b/include/uapi/linux/bpf.h
>>>> @@ -3836,6 +3836,12 @@ union bpf_attr {
>>>>     *	Return
>>>>     *		A pointer to a struct socket on success or NULL if the file is
>>>>     *		not a socket.
>>>> + *
>>>> + * long bpf_redirect_xsk(struct xdp_buff *xdp_md, u64 action)
>>>> + *	Description
>>>> + *		Redirect to the registered AF_XDP socket.
>>>> + *	Return
>>>> + *		**XDP_REDIRECT** on success, otherwise the action parameter is returned.
>>>>     */
>>>
>>> I think it would be better to make the second argument a 'flags'
>>> argument and make values > XDP_TX invalid (like we do in
>>> bpf_xdp_redirect_map() now). By allowing any value as return you lose
>>> the ability to turn it into a flags argument later...
>>>
>>
>> Yes, but that adds a run-time check. I prefer this non-checked version,
>> even though it is a bit less futureproof.
> 
> That...seems a bit short-sighted? :)
> Can you actually see a difference in your performance numbers?
>

I would rather add an additional helper *if* we see the need for flags,
instead of paying for that upfront. For me, BPF is about being able to
specialize, and not having one call with tons of checks.

(Related; Going forward, the growing switch() for redirect targets in
xdp_do_redirect() is a concern for me...)

And yes, even with all those fancy branch predictors, less instructions
is still less. :-) (It shows in my ubenchs.)


Björn