netdev - Re: [PATCH RFCv2 bpf-next 17/18] xsk: AF_XDP xdp-hints support in desc options

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Date:   Fri, 9 Sep 2022 09:12:38 +0100
From:   Maryam Tahhan <mtahhan@...hat.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
        xdp-hints@...-project.net, larysa.zaremba@...el.com,
        memxor@...il.com, Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        dave@...cker.co.uk, Magnus Karlsson <magnus.karlsson@...el.com>,
        bjorn@...nel.org
Subject: Re: [PATCH RFCv2 bpf-next 17/18] xsk: AF_XDP xdp-hints support in
 desc options

<snip>
>>>>
>>>> * Instead encode this information into each metadata entry in the
>>>> metadata area, in some way so that a flags field is not needed (-1
>>>> signifies not valid, or whatever happens to make sense). This has the
>>>> drawback that the user might have to look at a large number of entries
>>>> just to find out there is nothing valid to read. To alleviate this, it
>>>> could be combined with the next suggestion.
>>>>
>>>> * Dedicate one bit in the options field to indicate that there is at
>>>> least one valid metadata entry in the metadata area. This could be
>>>> combined with the two approaches above. However, depending on what
>>>> metadata you have enabled, this bit might be pointless. If some
>>>> metadata is always valid, then it serves no purpose. But it might if
>>>> all enabled metadata is rarely valid, e.g., if you get an Rx timestamp
>>>> on one packet out of one thousand.
>>>>
>>
>> I like this option better! Except that I have hoped to get 2 bits ;-)
> 
> I will give you two if you need it Jesper, no problem :-).
> 

Ok I will look at implementing and testing this and post an update.

Thanks folks

>> The performance advantage is that the AF_XDP descriptor bits will
>> already be cache-hot, and if it indicates no-metadata-hints the AF_XDP
>> application can avoid reading the metadata cache-line :-).
> 
> Agreed. I prefer if we can keep it simple and fast like this.
> 
<snip>