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

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

On Tue, Apr 9, 2019 at 11:11 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 04/09/2019 08:44 AM, 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
>
> Hmm, bit unfortunate, but I can see why it's needed inside the header here
> as it's installed into the system and needs to be included by applications.
> Fine with me. In any case, I still think we should also fix it for tooling
> infra to avoid others running into the same issue, lemme resurrect [0] from
> back in the days.
>
> One question though regarding above arm32 implementation. You match these
> barriers against the ones from af_xdp in kernel. So smp_* variants from the
> arch/arm/include/asm/barrier.h header need to be matched. This selects the
> dmb(ish) and dmb(ishst) variant. The implementation of dmb() however is
> quite CPU dependent, the above assumes to match with __LINUX_ARM_ARCH__ >= 7.
> Perhaps this assumption needs at minimum a comment to make it clear for
> developers.

Good idea. I will put a comment in there to clarify this.

/Magnus

>   [0] https://lore.kernel.org/netdev/20181017144156.16639-2-daniel@iogearbox.net/
>
> >  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;
> >
>