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