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 at 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
