[dpdk-dev] [PATCH] timer: add lfence before TSC read

[Didier Pallard <didier.pallard@6wind.com>]

According to Intel Developer's Manual:

"The RDTSC instruction is not a serializing instruction. It does not necessarily wait
 until all previous instructions have been executed before reading the counter. Simi-
 larly, subsequent instructions may begin execution before the read operation is
 performed. If software requires RDTSC to be executed only after all previous instruc-
 tions have completed locally, it can either use RDTSCP (if the processor supports that
 instruction) or execute the sequence LFENCE;RDTSC."

So add a lfence instruction before rdtsc to synchronize read operations and ensure
that the read is done at the expected instant.

Signed-off-by: Didier Pallard <didier.pallard@6wind.com>
---
 lib/librte_eal/common/include/rte_cycles.h |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/librte_eal/common/include/rte_cycles.h b/lib/librte_eal/common/include/rte_cycles.h
index cc6fe71..487dba6 100644
--- a/lib/librte_eal/common/include/rte_cycles.h
+++ b/lib/librte_eal/common/include/rte_cycles.h
@@ -110,6 +110,9 @@ rte_rdtsc(void)
 		};
 	} tsc;
 
+	/* serialize previous load instructions in pipe */
+	asm volatile("lfence");
+
 #ifdef RTE_LIBRTE_EAL_VMWARE_TSC_MAP_SUPPORT
 	if (unlikely(rte_cycles_vmware_tsc_map)) {
 		/* ecx = 0x10000 corresponds to the physical TSC for VMware */
-- 
1.7.10.4