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

[Magnus Karlsson <magnus.karlsson@gmail.com>]

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.

> Thanks,
> Daniel