Re: [PATCH bpf 2/2] libbpf: remove dependency on barrier.h in xsk.h

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

On Wed, Sep 4, 2019 at 7:32 AM Yauheni Kaliuta
<yauheni.kaliuta@redhat.com> wrote:
>
> Hi, Magnus!
>
> >>>>> On Tue,  9 Apr 2019 08:44:13 +0200, Magnus Karlsson  wrote:
>
>  > The use of smp_rmb() and smp_wmb() creates a Linux header dependency
>  > on barrier.h that is uneccessary in most parts. This patch implements
>  > the two small defines that are needed from barrier.h. As a bonus, the
>  > new implementations are faster than the default ones as they default
>  > to sfence and lfence for x86, while we only need a compiler barrier in
>  > our case. Just as it is when the same ring access code is compiled in
>  > the kernel.
>
>  > Fixes: 1cad07884239 ("libbpf: add support for using AF_XDP sockets")
>  > Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
>  > ---
>  >  tools/lib/bpf/xsk.h | 19 +++++++++++++++++--
>  >  1 file changed, 17 insertions(+), 2 deletions(-)
>
>  > diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
>  > index 3638147..317b44f 100644
>  > --- a/tools/lib/bpf/xsk.h
>  > +++ b/tools/lib/bpf/xsk.h
>  > @@ -39,6 +39,21 @@ DEFINE_XSK_RING(xsk_ring_cons);
>  >  struct xsk_umem;
>  >  struct xsk_socket;
>
>  > +#if !defined bpf_smp_rmb && !defined bpf_smp_wmb
>  > +# if defined(__i386__) || defined(__x86_64__)
>  > +#  define bpf_smp_rmb() asm volatile("" : : : "memory")
>  > +#  define bpf_smp_wmb() asm volatile("" : : : "memory")
>  > +# elif defined(__aarch64__)
>  > +#  define bpf_smp_rmb() asm volatile("dmb ishld" : : : "memory")
>  > +#  define bpf_smp_wmb() asm volatile("dmb ishst" : : : "memory")
>  > +# elif defined(__arm__)
>  > +#  define bpf_smp_rmb() asm volatile("dmb ish" : : : "memory")
>  > +#  define bpf_smp_wmb() asm volatile("dmb ishst" : : : "memory")
>  > +# else
>  > +#  error Architecture not supported by the XDP socket code in libbpf.
>  > +# endif
>  > +#endif
>  > +
>
> What about other architectures then?

AF_XDP has not been tested on anything else, as far as I know. But
contributions that extend it to more archs are very welcome.

/Magnus

>
>  >  static inline __u64 *xsk_ring_prod__fill_addr(struct xsk_ring_prod *fill,
>  >                                            __u32 idx)
>  >  {
>  > @@ -119,7 +134,7 @@ static inline void xsk_ring_prod__submit(struct xsk_ring_prod *prod, size_t nb)
>  >      /* Make sure everything has been written to the ring before signalling
>  >       * this to the kernel.
>  >       */
>  > -    smp_wmb();
>  > +    bpf_smp_wmb();
>
>  >      *prod->producer += nb;
>  >  }
>  > @@ -133,7 +148,7 @@ static inline size_t xsk_ring_cons__peek(struct xsk_ring_cons *cons,
>  >              /* Make sure we do not speculatively read the data before
>  >               * we have received the packet buffers from the ring.
>  >               */
>  > -            smp_rmb();
>  > +            bpf_smp_rmb();
>
>  >              *idx = cons->cached_cons;
>  cons-> cached_cons += entries;
>  > --
>  > 2.7.4
>
>
> --
> WBR,
> Yauheni Kaliuta