[dpdk-dev] [PATCH v3 2/2] eal/x86: Use lock-prefixed instructions to reduce cost of rte_smp_mb()

[Konstantin Ananyev <konstantin.ananyev@intel.com>]

On x86 it  is possible to use lock-prefixed instructions to get
the similar effect as mfence.
As pointed by Java guys, on most modern HW that gives a better
performance than using mfence:
https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
That patch adopts that technique for rte_smp_mb() implementation.
On BDW 2.2 mb_autotest on single lcore reports 2X cycle reduction,
i.e. from ~110 to ~55 cycles per operation.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
---
 .../common/include/arch/x86/rte_atomic.h           | 44 +++++++++++++++++++++-
 1 file changed, 42 insertions(+), 2 deletions(-)

diff --git a/lib/librte_eal/common/include/arch/x86/rte_atomic.h b/lib/librte_eal/common/include/arch/x86/rte_atomic.h
index 8469f97e1..9d466d94a 100644
--- a/lib/librte_eal/common/include/arch/x86/rte_atomic.h
+++ b/lib/librte_eal/common/include/arch/x86/rte_atomic.h
@@ -26,12 +26,52 @@ extern "C" {
 
 #define	rte_rmb() _mm_lfence()
 
-#define rte_smp_mb() rte_mb()
-
 #define rte_smp_wmb() rte_compiler_barrier()
 
 #define rte_smp_rmb() rte_compiler_barrier()
 
+/*
+ * From Intel Software Development Manual; Vol 3;
+ * 8.2.2 Memory Ordering in P6 and More Recent Processor Families:
+ * ...
+ * . Reads are not reordered with other reads.
+ * . Writes are not reordered with older reads.
+ * . Writes to memory are not reordered with other writes,
+ *   with the following exceptions:
+ *   . streaming stores (writes) executed with the non-temporal move
+ *     instructions (MOVNTI, MOVNTQ, MOVNTDQ, MOVNTPS, and MOVNTPD); and
+ *   . string operations (see Section 8.2.4.1).
+ *  ...
+ * . Reads may be reordered with older writes to different locations but not
+ * with older writes to the same location.
+ * . Reads or writes cannot be reordered with I/O instructions,
+ * locked instructions, or serializing instructions.
+ * . Reads cannot pass earlier LFENCE and MFENCE instructions.
+ * . Writes ... cannot pass earlier LFENCE, SFENCE, and MFENCE instructions.
+ * . LFENCE instructions cannot pass earlier reads.
+ * . SFENCE instructions cannot pass earlier writes ...
+ * . MFENCE instructions cannot pass earlier reads, writes ...
+ *
+ * As pointed by Java guys, that makes possible to use lock-prefixed
+ * instructions to get the same effect as mfence and on most modern HW
+ * that gives a better perfomance then using mfence:
+ * https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
+ * Basic idea is to use lock prefixed add with some dummy memory location
+ * as the destination. From their experiments 128B(2 cache lines) below
+ * current stack pointer looks like a good candidate.
+ * So below we use that techinque for rte_smp_mb() implementation.
+ */
+
+static __rte_always_inline void
+rte_smp_mb(void)
+{
+#ifdef RTE_ARCH_I686
+	asm volatile("lock addl $0, -128(%%esp); " ::: "memory");
+#else
+	asm volatile("lock addl $0, -128(%%rsp); " ::: "memory");
+#endif
+}
+
 #define rte_io_mb() rte_mb()
 
 #define rte_io_wmb() rte_compiler_barrier()
-- 
2.13.6