netdev - Re: [PATCH bpf-next v3 00/15] Introducing AF

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Date:   Thu, 17 May 2018 20:38:34 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Netdev <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        <michael.lundkvist@...csson.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>
Subject: Re: [PATCH bpf-next v3 00/15] Introducing AF_XDP support

On 5/16/18 11:46 PM, Björn Töpel wrote:
> 2018-05-04 1:38 GMT+02:00 Alexei Starovoitov <alexei.starovoitov@...il.com>:
>> On Fri, May 04, 2018 at 12:49:09AM +0200, Daniel Borkmann wrote:
>>> On 05/02/2018 01:01 PM, Björn Töpel wrote:
>>>> From: Björn Töpel <bjorn.topel@...el.com>
>>>>
>>>> This patch set introduces a new address family called AF_XDP that is
>>>> optimized for high performance packet processing and, in upcoming
>>>> patch sets, zero-copy semantics. In this patch set, we have removed
>>>> all zero-copy related code in order to make it smaller, simpler and
>>>> hopefully more review friendly. This patch set only supports copy-mode
>>>> for the generic XDP path (XDP_SKB) for both RX and TX and copy-mode
>>>> for RX using the XDP_DRV path. Zero-copy support requires XDP and
>>>> driver changes that Jesper Dangaard Brouer is working on. Some of his
>>>> work has already been accepted. We will publish our zero-copy support
>>>> for RX and TX on top of his patch sets at a later point in time.
>>>
>>> +1, would be great to see it land this cycle. Saw few minor nits here
>>> and there but nothing to hold it up, for the series:
>>>
>>> Acked-by: Daniel Borkmann <daniel@...earbox.net>
>>>
>>> Thanks everyone!
>>
>> Great stuff!
>>
>> Applied to bpf-next, with one condition.
>> Upcoming zero-copy patches for both RX and TX need to be posted
>> and reviewed within this release window.
>> If netdev community as a whole won't be able to agree on the zero-copy
>> bits we'd need to revert this feature before the next merge window.
>>
>> Few other minor nits:
>> patch 3:
>> +struct xdp_ring {
>> +       __u32 producer __attribute__((aligned(64)));
>> +       __u32 consumer __attribute__((aligned(64)));
>> +};
>> It kinda begs for ____cacheline_aligned_in_smp to be introduced for uapi headers.
>>
>
> Hmm, I need some guidance on what a sane uapi variant would be. We
> can't have the uapi depend on the kernel build. ARM64, e.g., can have
> both 64B and 128B according to the specs. Contemporary IA processors
> have 64B.
>
> The simplest, and maybe most future-proof, would be 128B aligned for
> all. Another is having 128B for ARM and 64B for all IA. A third option
> is having a hand-shaking API (I think virtio has that) for determine
> the cache line size, but I'd rather not go down that route.
>
> Thoughts/ideas on how a uapi ____cacheline_aligned_in_smp version
> would look like?

I suspect i40e+arm combination wasn't tested anyway.
The api may have endianness issues too on something like sparc.
I think the way to be backwards compatible in this area
is to make the api usable on x86 only by adding
to include/uapi/linux/if_xdp.h
#if defined(__x86_64__)
#define AF_XDP_CACHE_BYTES 64
#else
#error "AF_XDP support is not yet available for this architecture"
#endif
and doing:
     __u32 producer __attribute__((aligned(AF_XDP_CACHE_BYTES)));
     __u32 consumer __attribute__((aligned(AF_XDP_CACHE_BYTES)));

And progressively add to this for arm64 and few other archs.
Eventually removing #error and adding some generic define
that's good enough for long tail of architectures that
we really cannot test.