netdev - Re: [PATCH bpf v2 2/2] libbpf: remove dependency on barrier.h in xsk.h

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Date:   Thu, 11 Apr 2019 09:54:18 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Y Song <ys114321@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        bruce.richardson@...el.com, ciara.loftus@...el.com,
        ilias.apalodimas@...aro.org, Ye Xiaolong <xiaolong.ye@...el.com>,
        ferruh.yigit@...el.com, "Zhang, Qi Z" <qi.z.zhang@...el.com>,
        georgmueller@....net
Subject: Re: [PATCH bpf v2 2/2] libbpf: remove dependency on barrier.h in xsk.h

On Wed, Apr 10, 2019 at 9:08 PM Y Song <ys114321@...il.com> wrote:
>
> On Wed, Apr 10, 2019 at 12:21 AM Magnus Karlsson
> <magnus.karlsson@...el.com> 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@...el.com>
> > ---
> >  tools/lib/bpf/xsk.h | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
> > index 3638147..69136d9 100644
> > --- a/tools/lib/bpf/xsk.h
> > +++ b/tools/lib/bpf/xsk.h
> > @@ -39,6 +39,22 @@ DEFINE_XSK_RING(xsk_ring_cons);
> >  struct xsk_umem;
> >  struct xsk_socket;
> >
> > +#if !defined bpf_smp_rmb && !defined bpf_smp_wmb
>
> Maybe add some comments to explain the different between bpf_smp_{r,w}mb
> and smp_{r,w}mb so later users will have a better idea which to pick?

Ouch, that is a hard one. I would just recommend people to read
Documentation/memory-barriers.txt. My attempt at explaining all this
would not be pretty and likely sprinkled with errors ;-).

> > +# 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__)
> > +/* These are only valid for armv7 and above */
> > +#  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
>
> Since this is generic enough and could be used by other files as well,
> maybe put it into libbpf_util.h?

Good question. Do not know. Daniel suggested introducing [0] and
perhaps that can be used by the broader libbpf code base? The
important part for this patch set is that these operations match the
ones in the kernel on the other end of the ring.

[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 +135,7 @@ 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 +149,7 @@ 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();
>
> Could you explain why a compiler barrier is good enough here on x86? Note that
> the load cons->cached_cons could be reordered with earlier
> non-overlapping stores
> at runtime.

The bpf_smp_rmb() is there to protect the data in the ring itself to
be read by the consumer before the producer has signaled that it has
finished “producing” them by updating the producer (head) pointer. As
stores are not reordered with other stores on x86 (nor loads with
other loads), the update of the producer pointer will always be
observed after the writing of the data in the ring, as that is done
before the update of the producer pointer in xsk_ring_prod__submit().
One side only updates and the other side only reads. cached_cons is a
local variable and only for operations done by another core can we
observe loads being reordered with older stores to different
locations. Since no one else is touching cached_cons, this will not
happen.

/Magnus

> >
> >                 *idx = cons->cached_cons;
> >                 cons->cached_cons += entries;
> > --
> > 2.7.4
> >