netdev - Re: [PATCH][V2] libbpf: add support for canceling cached

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Date:   Wed, 25 Nov 2020 13:22:52 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Li RongQing <lirongqing@...du.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [PATCH][V2] libbpf: add support for canceling cached_cons advance

On 11/25/20 11:09 AM, Magnus Karlsson wrote:
> On Wed, Nov 25, 2020 at 11:07 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> On 11/25/20 10:13 AM, Magnus Karlsson wrote:
>>> On Wed, Nov 25, 2020 at 10:02 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>> On 11/25/20 9:30 AM, Magnus Karlsson wrote:
>>>>> On Tue, Nov 24, 2020 at 10:58 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>>>> On 11/24/20 9:12 AM, Magnus Karlsson wrote:
>>>>>>> On Tue, Nov 24, 2020 at 8:33 AM Li RongQing <lirongqing@...du.com> wrote:
>>>>>>>>
>>>>>>>> Add a new function for returning descriptors the user received
>>>>>>>> after an xsk_ring_cons__peek call. After the application has
>>>>>>>> gotten a number of descriptors from a ring, it might not be able
>>>>>>>> to or want to process them all for various reasons. Therefore,
>>>>>>>> it would be useful to have an interface for returning or
>>>>>>>> cancelling a number of them so that they are returned to the ring.
>>>>>>>>
>>>>>>>> This patch adds a new function called xsk_ring_cons__cancel that
>>>>>>>> performs this operation on nb descriptors counted from the end of
>>>>>>>> the batch of descriptors that was received through the peek call.
>>>>>>>>
>>>>>>>> Signed-off-by: Li RongQing <lirongqing@...du.com>
>>>>>>>> [ Magnus Karlsson: rewrote changelog ]
>>>>>>>> Cc: Magnus Karlsson <magnus.karlsson@...el.com>
>>>>>>>> ---
>>>>>>>> diff with v1: fix the building, and rewrote changelog
>>>>>>>>
>>>>>>>>      tools/lib/bpf/xsk.h | 6 ++++++
>>>>>>>>      1 file changed, 6 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
>>>>>>>> index 1069c46364ff..1719a327e5f9 100644
>>>>>>>> --- a/tools/lib/bpf/xsk.h
>>>>>>>> +++ b/tools/lib/bpf/xsk.h
>>>>>>>> @@ -153,6 +153,12 @@ static inline size_t xsk_ring_cons__peek(struct xsk_ring_cons *cons,
>>>>>>>>             return entries;
>>>>>>>>      }
>>>>>>>>
>>>>>>>> +static inline void xsk_ring_cons__cancel(struct xsk_ring_cons *cons,
>>>>>>>> +                                        size_t nb)
>>>>>>>> +{
>>>>>>>> +       cons->cached_cons -= nb;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>      static inline void xsk_ring_cons__release(struct xsk_ring_cons *cons, size_t nb)
>>>>>>>>      {
>>>>>>>>             /* Make sure data has been read before indicating we are done
>>>>>>>> --
>>>>>>>> 2.17.3
>>>>>>>
>>>>>>> Thank you RongQing.
>>>>>>>
>>>>>>> Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
>>>>>>
>>>>>> @Magnus: shouldn't the xsk_ring_cons__cancel() nb type be '__u32 nb' instead?
>>>>>
>>>>> All the other interfaces have size_t as the type for "nb". It is kind
>>>>> of weird as a __u32 would have made more sense, but cannot actually
>>>>> remember why I chose a size_t two years ago. But for consistency with
>>>>> the other interfaces, let us keep it a size_t for now. I will do some
>>>>> research around the reason.
>>>>
>>>> It's actually a bit of a mix currently which is what got me confused:
>>>>
>>>> static inline __u32 xsk_prod_nb_free(struct xsk_ring_prod *r, __u32 nb)
>>>> static inline __u32 xsk_cons_nb_avail(struct xsk_ring_cons *r, __u32 nb)
>>>> static inline size_t xsk_ring_prod__reserve(struct xsk_ring_prod *prod, size_t nb, __u32 *idx)
>>>> static inline void xsk_ring_prod__submit(struct xsk_ring_prod *prod, size_t nb)
>>>> static inline size_t xsk_ring_cons__peek(struct xsk_ring_cons *cons, size_t nb, __u32 *idx)
>>>> static inline void xsk_ring_cons__release(struct xsk_ring_cons *cons, size_t nb)
>>>>
>>>> (I can take it in as-is, but would be nice to clean it up a bit to avoid confusion.)
>>>
>>> Hmm, that is confusing indeed. Well, the best choice would be __u32
>>> everywhere since the ring pointers themselves are __u32. But I am
>>> somewhat afraid of changing an API. Can we guarantee that a change
>>> from size_t to __u32 will not break some user's compilation? Another
>>> option would be to clean this up next year when we will very likely
>>> produce a 1.0 version of this API and at that point we can change some
>>> things. What do you think would be the best approach?
>>
>> Given they're all inlines, imho, risk should be fairly low to switch all to __u32.
>> I would probably go and verify first with DPDK as main user of the lib and/or write
>> some test cases to see if compiler spills any new warnings and the like, but if not
>> the case then we should do it for bpf-next so this has plenty of exposure in the
>> meantime. Any nb large than u32 max is a bug in any case.
> 
> Sounds good. Will do and get back to you.

Great, thanks, I took in the current patch to bpf-next in that case and the rest can
be followed-up as discussed.

Thanks,
Daniel