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

[Daniel Borkmann <daniel@iogearbox.net>]

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.)

Thanks,
Daniel