Currently this test does a single CLFLUSH on its memory location
but due to speculative execution this might not cause LLC misses.
Instead, do a cache flush on each loop iteration to confuse the prediction
and make sure that cache misses always occur.
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
.../selftests/kvm/x86_64/pmu_counters_test.c | 20 +++++++++----------
1 file changed, 9 insertions(+), 11 deletions(-)
diff --git a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
index 96446134c00b7..ddc0b7e4a888e 100644
--- a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
+++ b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
@@ -14,8 +14,8 @@
* instructions that are needed to set up the loop and then disabled the
* counter. 1 CLFLUSH/CLFLUSHOPT/NOP, 1 MFENCE, 2 MOV, 2 XOR, 1 WRMSR.
*/
-#define NUM_EXTRA_INSNS 7
-#define NUM_INSNS_RETIRED (NUM_BRANCHES + NUM_EXTRA_INSNS)
+#define NUM_EXTRA_INSNS 5
+#define NUM_INSNS_RETIRED (NUM_BRANCHES * 2 + NUM_EXTRA_INSNS)
static uint8_t kvm_pmu_version;
static bool kvm_has_perf_caps;
@@ -133,9 +133,8 @@ static void guest_assert_event_count(uint8_t idx,
* doesn't need to be clobbered as the input value, @pmc_msr, is restored
* before the end of the sequence.
*
- * If CLFUSH{,OPT} is supported, flush the cacheline containing (at least) the
- * start of the loop to force LLC references and misses, i.e. to allow testing
- * that those events actually count.
+ * If CLFUSH{,OPT} is supported, flush the cacheline containing the CLFUSH{,OPT}
+ * instruction on each loop iteration to ensure that LLC cache misses happen.
*
* If forced emulation is enabled (and specified), force emulation on a subset
* of the measured code to verify that KVM correctly emulates instructions and
@@ -145,10 +144,9 @@ static void guest_assert_event_count(uint8_t idx,
#define GUEST_MEASURE_EVENT(_msr, _value, clflush, FEP) \
do { \
__asm__ __volatile__("wrmsr\n\t" \
- clflush "\n\t" \
- "mfence\n\t" \
- "1: mov $" __stringify(NUM_BRANCHES) ", %%ecx\n\t" \
- FEP "loop .\n\t" \
+ " mov $" __stringify(NUM_BRANCHES) ", %%ecx\n\t" \
+ "1: " clflush "\n\t" \
+ FEP "loop 1b\n\t" \
FEP "mov %%edi, %%ecx\n\t" \
FEP "xor %%eax, %%eax\n\t" \
FEP "xor %%edx, %%edx\n\t" \
@@ -163,9 +161,9 @@ do { \
wrmsr(pmc_msr, 0); \
\
if (this_cpu_has(X86_FEATURE_CLFLUSHOPT)) \
- GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflushopt 1f", FEP); \
+ GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflushopt .", FEP); \
else if (this_cpu_has(X86_FEATURE_CLFLUSH)) \
- GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflush 1f", FEP); \
+ GUEST_MEASURE_EVENT(_ctrl_msr, _value, "clflush .", FEP); \
else \
GUEST_MEASURE_EVENT(_ctrl_msr, _value, "nop", FEP); \
\
--
2.26.3
