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Date:   Wed, 13 Feb 2019 12:49:35 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Magnus Karlsson" <magnus.karlsson@...il.com>
Cc:     "Magnus Karlsson" <magnus.karlsson@...el.com>,
        "Björn Töpel" <bjorn.topel@...el.com>,
        ast@...nel.org, "Daniel Borkmann" <daniel@...earbox.net>,
        "Network Development" <netdev@...r.kernel.org>,
        "Jakub Kicinski" <jakub.kicinski@...ronome.com>,
        "Björn Töpel" <bjorn.topel@...il.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>, xiaolong.ye@...el.com
Subject: Re: [PATCH bpf-next v4 0/2] libbpf: adding AF_XDP support

On 13 Feb 2019, at 3:32, Magnus Karlsson wrote:

> On Mon, Feb 11, 2019 at 9:44 PM Jonathan Lemon 
> <jonathan.lemon@...il.com> wrote:
>>
>> On 8 Feb 2019, at 5:05, Magnus Karlsson wrote:
>>
>>> This patch proposes to add AF_XDP support to libbpf. The main reason
>>> for this is to facilitate writing applications that use AF_XDP by
>>> offering higher-level APIs that hide many of the details of the 
>>> AF_XDP
>>> uapi. This is in the same vein as libbpf facilitates XDP adoption by
>>> offering easy-to-use higher level interfaces of XDP
>>> functionality. Hopefully this will facilitate adoption of AF_XDP, 
>>> make
>>> applications using it simpler and smaller, and finally also make it
>>> possible for applications to benefit from optimizations in the 
>>> AF_XDP
>>> user space access code. Previously, people just copied and pasted 
>>> the
>>> code from the sample application into their application, which is 
>>> not
>>> desirable.
>>
>> I like the idea of encapsulating the boilerplate logic in a library.
>>
>> I do think there is an important missing piece though - there should 
>> be
>> some code which queries the netdev for how many queues are attached, 
>> and
>> create the appropriate number of umem/AF_XDP sockets.
>>
>> I ran into this issue when testing the current AF_XDP code - on my 
>> test
>> boxes, the mlx5 card has 55 channels (aka queues), so when the test 
>> program
>> binds only to channel 0, nothing works as expected, since not all 
>> traffic
>> is being intercepted.  While obvious in hindsight, this took a while 
>> to
>> track down.
>
> Yes, agreed. You are not the first one to stumble upon this problem
> :-). Let me think a little bit on how to solve this in a good way. We
> need this to be simple and intuitive, as you say.

Has any investigation been done on using some variant of MPSC 
implementation
as an intermediate form for AF_XDP?  E.g.: something like LCRQ or the 
bulkQ
in bpf devmap/cpumap.  I'm aware that this would be slightly slower, as 
it
would introduce a lock in the path, but I'd think that having DEVMAP, 
CPUMAP
and XSKMAP all behave the same way would add more flexibility.

Ideally, if the configuration matches the underlying hardware, then the
implementation would reduce to the current setup (and allow ZC 
implementations),
but a non-matching configuration would still work - as opposed to the 
current
situation.
-- 
Jonathan

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