Re: [kvm-unit-tests Patch v3 09/11] x86: pmu: Improve LLC misses event verification

From: Mi, Dapeng
Date: Wed Mar 27 2024 - 05:18:19 EST



On 3/27/2024 2:23 PM, Mingwei Zhang wrote:
On Wed, Jan 03, 2024, Dapeng Mi wrote:
When running pmu test on SPR, sometimes the following failure is
reported.

1 <= 0 <= 1000000
FAIL: Intel: llc misses-4

Currently The LLC misses occurring only depends on probability. It's
possible that there is no LLC misses happened in the whole loop(),
especially along with processors have larger and larger cache size just
like what we observed on SPR.

Thus, add clflush instruction into the loop() asm blob and ensure once
LLC miss is triggered at least.

Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
I wonder if we can skip all LLC tests if CPU does not have
clflush/clflushopt properties?

Looks reasonable, the LLC miss event should be skipped at least if clflush/clflushopt are not supported since the validation can't grantee the correctness. But for LLC reference event, I think we can keep it since the asm_loop() would always 1 LLC reference at least.

If no disapproval, I would change code to skip LLC miss event if clflush/clflushopt are not supported in next version. Thanks.


---
x86/pmu.c | 43 ++++++++++++++++++++++++++++++-------------
1 file changed, 30 insertions(+), 13 deletions(-)

diff --git a/x86/pmu.c b/x86/pmu.c
index b764827c1c3d..8fd3db0fbf81 100644
--- a/x86/pmu.c
+++ b/x86/pmu.c
@@ -20,19 +20,21 @@
// Instrustion number of LOOP_ASM code
#define LOOP_INSTRNS 10
-#define LOOP_ASM \
+#define LOOP_ASM(_clflush) \
+ _clflush "\n\t" \
+ "mfence;\n\t" \
"1: mov (%1), %2; add $64, %1;\n\t" \
"nop; nop; nop; nop; nop; nop; nop;\n\t" \
"loop 1b;\n\t"
-/*Enable GLOBAL_CTRL + disable GLOBAL_CTRL instructions */
-#define PRECISE_EXTRA_INSTRNS (2 + 4)
+/*Enable GLOBAL_CTRL + disable GLOBAL_CTRL + clflush/mfence instructions */
+#define PRECISE_EXTRA_INSTRNS (2 + 4 + 2)
#define PRECISE_LOOP_INSTRNS (N * LOOP_INSTRNS + PRECISE_EXTRA_INSTRNS)
#define PRECISE_LOOP_BRANCHES (N)
-#define PRECISE_LOOP_ASM \
+#define PRECISE_LOOP_ASM(_clflush) \
"wrmsr;\n\t" \
"mov %%ecx, %%edi; mov %%ebx, %%ecx;\n\t" \
- LOOP_ASM \
+ LOOP_ASM(_clflush) \
"mov %%edi, %%ecx; xor %%eax, %%eax; xor %%edx, %%edx;\n\t" \
"wrmsr;\n\t"
@@ -72,14 +74,30 @@ char *buf;
static struct pmu_event *gp_events;
static unsigned int gp_events_size;
+#define _loop_asm(_clflush) \
+do { \
+ asm volatile(LOOP_ASM(_clflush) \
+ : "=c"(tmp), "=r"(tmp2), "=r"(tmp3) \
+ : "0"(N), "1"(buf)); \
+} while (0)
+
+#define _precise_loop_asm(_clflush) \
+do { \
+ asm volatile(PRECISE_LOOP_ASM(_clflush) \
+ : "=b"(tmp), "=r"(tmp2), "=r"(tmp3) \
+ : "a"(eax), "d"(edx), "c"(global_ctl), \
+ "0"(N), "1"(buf) \
+ : "edi"); \
+} while (0)
static inline void __loop(void)
{
unsigned long tmp, tmp2, tmp3;
- asm volatile(LOOP_ASM
- : "=c"(tmp), "=r"(tmp2), "=r"(tmp3)
- : "0"(N), "1"(buf));
+ if (this_cpu_has(X86_FEATURE_CLFLUSH))
+ _loop_asm("clflush (%1)");
+ else
+ _loop_asm("nop");
}
/*
@@ -96,11 +114,10 @@ static inline void __precise_count_loop(u64 cntrs)
u32 eax = cntrs & (BIT_ULL(32) - 1);
u32 edx = cntrs >> 32;
- asm volatile(PRECISE_LOOP_ASM
- : "=b"(tmp), "=r"(tmp2), "=r"(tmp3)
- : "a"(eax), "d"(edx), "c"(global_ctl),
- "0"(N), "1"(buf)
- : "edi");
+ if (this_cpu_has(X86_FEATURE_CLFLUSH))
+ _precise_loop_asm("clflush (%1)");
+ else
+ _precise_loop_asm("nop");
}
static inline void loop(u64 cntrs)
--
2.34.1