netdev - Re: [PATCH bpf 2/2] libbpf: remove dependency on barrier.h in xsk.h

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Date:   Wed, 10 Apr 2019 00:28:34 +0200
From:   Georg Müller <georgmueller@....net>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf@...r.kernel.org, bruce.richardson@...el.com,
        ciara.loftus@...el.com, ilias.apalodimas@...aro.org,
        Ye Xiaolong <xiaolong.ye@...el.com>, ferruh.yigit@...el.com,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>, will.deacon@....com
Subject: Re: [PATCH bpf 2/2] libbpf: remove dependency on barrier.h in xsk.h

Am 09.04.19 um 13:29 schrieb Magnus Karlsson:
> On Tue, Apr 9, 2019 at 11:11 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> On 04/09/2019 08:44 AM, Magnus Karlsson wrote:
>>> The use of smp_rmb() and smp_wmb() creates a Linux header dependency
>>> on barrier.h that is uneccessary in most parts. This patch implements
>>> the two small defines that are needed from barrier.h. As a bonus, the
>>> new implementations are faster than the default ones as they default
>>> to sfence and lfence for x86, while we only need a compiler barrier in
>>> our case. Just as it is when the same ring access code is compiled in
>>> the kernel.
>>>
>>> Fixes: 1cad07884239 ("libbpf: add support for using AF_XDP sockets")
>>> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
>>> ---
>>>  tools/lib/bpf/xsk.h | 19 +++++++++++++++++--
>>>  1 file changed, 17 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
>>> index 3638147..317b44f 100644
>>> --- a/tools/lib/bpf/xsk.h
>>> +++ b/tools/lib/bpf/xsk.h
>>> @@ -39,6 +39,21 @@ DEFINE_XSK_RING(xsk_ring_cons);
>>>  struct xsk_umem;
>>>  struct xsk_socket;
>>>
>>> +#if !defined bpf_smp_rmb && !defined bpf_smp_wmb
>>> +# if defined(__i386__) || defined(__x86_64__)
>>> +#  define bpf_smp_rmb() asm volatile("" : : : "memory")
>>> +#  define bpf_smp_wmb() asm volatile("" : : : "memory")
>>> +# elif defined(__aarch64__)
>>> +#  define bpf_smp_rmb() asm volatile("dmb ishld" : : : "memory")
>>> +#  define bpf_smp_wmb() asm volatile("dmb ishst" : : : "memory")
>>> +# elif defined(__arm__)
>>> +#  define bpf_smp_rmb() asm volatile("dmb ish" : : : "memory")
>>> +#  define bpf_smp_wmb() asm volatile("dmb ishst" : : : "memory")
>>> +# else
>>> +#  error Architecture not supported by the XDP socket code in libbpf.
>>> +# endif
>>> +#endif
>>
>> Hmm, bit unfortunate, but I can see why it's needed inside the header here
>> as it's installed into the system and needs to be included by applications.
>> Fine with me. In any case, I still think we should also fix it for tooling
>> infra to avoid others running into the same issue, lemme resurrect [0] from
>> back in the days.
>>
>> One question though regarding above arm32 implementation. You match these
>> barriers against the ones from af_xdp in kernel. So smp_* variants from the
>> arch/arm/include/asm/barrier.h header need to be matched. This selects the
>> dmb(ish) and dmb(ishst) variant. The implementation of dmb() however is
>> quite CPU dependent, the above assumes to match with __LINUX_ARM_ARCH__ >= 7.
>> Perhaps this assumption needs at minimum a comment to make it clear for
>> developers.
>
> Good idea. I will put a comment in there to clarify this.
>
> /Magnus


Since this is a userspace library, what about using the C11 atomic_thread_fence()
from stdatomic.h - wouldn't that be sufficient?


>>   [0] https://lore.kernel.org/netdev/20181017144156.16639-2-daniel@iogearbox.net/
>>
>>>  static inline __u64 *xsk_ring_prod__fill_addr(struct xsk_ring_prod *fill,
>>>                                             __u32 idx)
>>>  {
>>> @@ -119,7 +134,7 @@ static inline void xsk_ring_prod__submit(struct xsk_ring_prod *prod, size_t nb)
>>>       /* Make sure everything has been written to the ring before signalling
>>>        * this to the kernel.
>>>        */
>>> -     smp_wmb();
>>> +     bpf_smp_wmb();
>>>
>>>       *prod->producer += nb;
>>>  }
>>> @@ -133,7 +148,7 @@ static inline size_t xsk_ring_cons__peek(struct xsk_ring_cons *cons,
>>>               /* Make sure we do not speculatively read the data before
>>>                * we have received the packet buffers from the ring.
>>>                */
>>> -             smp_rmb();
>>> +             bpf_smp_rmb();
>>>
>>>               *idx = cons->cached_cons;
>>>               cons->cached_cons += entries;
>>>
>>