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

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Date:   Wed, 25 Nov 2020 10:13:10 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 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?

> Thanks,
> Daniel