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

From: Yauheni Kaliuta <[email-masked]>
Date: Wed, 04 Sep 2019 08:32:06 +0300

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 <[email-masked]>
 > ---
 >  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?

 >  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