Re: [PATCH][V2] libbpf: add support for canceling cached_cons advance

[Daniel Borkmann <daniel@iogearbox.net>]

On 11/25/20 10:13 AM, Magnus Karlsson wrote:
> On Wed, Nov 25, 2020 at 10:02 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 11/25/20 9:30 AM, Magnus Karlsson wrote:
>>> On Tue, Nov 24, 2020 at 10:58 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>> On 11/24/20 9:12 AM, Magnus Karlsson wrote:
>>>>> On Tue, Nov 24, 2020 at 8:33 AM Li RongQing <lirongqing@baidu.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@baidu.com>
>>>>>> [ Magnus Karlsson: rewrote changelog ]
>>>>>> Cc: Magnus Karlsson <magnus.karlsson@intel.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@intel.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.

Thanks,
Daniel