From 2c5935f1b2b642cee8e1562396ec8a7781fc4c6d Mon Sep 17 00:00:00 2001 From: Magnus Karlsson Date: Tue, 16 Apr 2019 14:58:12 +0200 Subject: [PATCH] libbpf: optimize barrier for XDP socket rings The full memory barrier in the XDP socket rings on the consumer side between the load of the data and the store of the consumer ring is there to protect the store from being executed before the load of the data. If this was allowed to happen, the producer might overwrite the data field with a new entry before the consumer got the chance to read it. On x86, stores are guaranteed not to be reordered with older loads, so it does not need a full memory barrier here. A compile time barrier would be enough. This patch introdcues a new primitive in libbpf_util.h that implements a new barrier type (libbpf_smp_rwmb) hindering stores to be reordered with older loads. It is then used in the XDP socket ring access code in libbpf to improve performance. Signed-off-by: Magnus Karlsson Signed-off-by: Alexei Starovoitov --- tools/lib/bpf/libbpf_util.h | 5 +++++ tools/lib/bpf/xsk.h | 2 +- 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/tools/lib/bpf/libbpf_util.h b/tools/lib/bpf/libbpf_util.h index 71fbb2898b87..172b707e007b 100644 --- a/tools/lib/bpf/libbpf_util.h +++ b/tools/lib/bpf/libbpf_util.h @@ -32,20 +32,25 @@ do { \ # define libbpf_smp_wmb() asm volatile("" : : : "memory") # define libbpf_smp_mb() \ asm volatile("lock; addl $0,-4(%%rsp)" : : : "memory", "cc") +/* Hinders stores to be observed before older loads. */ +# define libbpf_smp_rwmb() asm volatile("" : : : "memory") #elif defined(__aarch64__) # define libbpf_smp_rmb() asm volatile("dmb ishld" : : : "memory") # define libbpf_smp_wmb() asm volatile("dmb ishst" : : : "memory") # define libbpf_smp_mb() asm volatile("dmb ish" : : : "memory") +# define libbpf_smp_rwmb() libbpf_smp_mb() #elif defined(__arm__) /* These are only valid for armv7 and above */ # define libbpf_smp_rmb() asm volatile("dmb ish" : : : "memory") # define libbpf_smp_wmb() asm volatile("dmb ishst" : : : "memory") # define libbpf_smp_mb() asm volatile("dmb ish" : : : "memory") +# define libbpf_smp_rwmb() libbpf_smp_mb() #else # warning Architecture missing native barrier functions in libbpf_util.h. # define libbpf_smp_rmb() __sync_synchronize() # define libbpf_smp_wmb() __sync_synchronize() # define libbpf_smp_mb() __sync_synchronize() +# define libbpf_smp_rwmb() __sync_synchronize() #endif #ifdef __cplusplus diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h index 2377c7a7f1b1..82ea71a0f3ec 100644 --- a/tools/lib/bpf/xsk.h +++ b/tools/lib/bpf/xsk.h @@ -152,7 +152,7 @@ static inline void xsk_ring_cons__release(struct xsk_ring_cons *cons, size_t nb) /* Make sure data has been read before indicating we are done * with the entries by updating the consumer pointer. */ - libbpf_smp_mb(); + libbpf_smp_rwmb(); *cons->consumer += nb; } -- 2.45.2