Hi Xiaoyao,
On 3/4/25 6:40 AM, Xiaoyao Li wrote:
> On 3/3/2025 6:00 AM, Dongli Zhang wrote:
>> When the PERFCORE is disabled with "-cpu host,-perfctr-core", it is
>> reflected in in guest dmesg.
>>
>> [ 0.285136] Performance Events: AMD PMU driver.
>
> I'm a little confused. wWhen no perfctr-core, AMD PMU driver can still be
> probed? (forgive me if I ask a silly question)
Intel use "cpuid -1 -l 0xa" to determine the support of PMU.
However, AMD doesn't use CPUID to determine PMU support (except AMD PMU
PerfMonV2).
I have derived everything from Linux kernel function amd_pmu_init().
As line 1521, the PMU isn't supported by old AMD CPUs.
1516 __init int amd_pmu_init(void)
1517 {
1518 int ret;
1519
1520 /* Performance-monitoring supported from K7 and later: */
1521 if (boot_cpu_data.x86 < 6)
1522 return -ENODEV;
1523
1524 x86_pmu = amd_pmu;
1525
1526 ret = amd_core_pmu_init();
1. Therefore, at least 4 PMCs are available (without 'perfctr-core').
2. With 'perfctr-core', there are 6 PMCs. (line 1410)
1404 static int __init amd_core_pmu_init(void)
1405 {
1406 union cpuid_0x80000022_ebx ebx;
1407 u64 even_ctr_mask = 0ULL;
1408 int i;
1409
1410 if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
1411 return 0;
1412
1413 /* Avoid calculating the value each time in the NMI handler */
1414 perf_nmi_window = msecs_to_jiffies(100);
1415
1416 /*
1417 * If core performance counter extensions exists, we must use
1418 * MSR_F15H_PERF_CTL/MSR_F15H_PERF_CTR msrs. See also
1419 * amd_pmu_addr_offset().
1420 */
1421 x86_pmu.eventsel = MSR_F15H_PERF_CTL;
1422 x86_pmu.perfctr = MSR_F15H_PERF_CTR;
1423 x86_pmu.cntr_mask64 = GENMASK_ULL(AMD64_NUM_COUNTERS_CORE
- 1, 0);
3. With PerfMonV2, extra global registers are available, as well as PMCs.
(line 1426)
1425 /* Check for Performance Monitoring v2 support */
1426 if (boot_cpu_has(X86_FEATURE_PERFMON_V2)) {
1427 ebx.full = cpuid_ebx(EXT_PERFMON_DEBUG_FEATURES);
1428
1429 /* Update PMU version for later usage */
1430 x86_pmu.version = 2;
1431
1432 /* Find the number of available Core PMCs */
1433 x86_pmu.cntr_mask64 =
GENMASK_ULL(ebx.split.num_core_pmc - 1, 0);
1434
1435 amd_pmu_global_cntr_mask = x86_pmu.cntr_mask64;
1436
1437 /* Update PMC handling functions */
1438 x86_pmu.enable_all = amd_pmu_v2_enable_all;
1439 x86_pmu.disable_all = amd_pmu_v2_disable_all;
1440 x86_pmu.enable = amd_pmu_v2_enable_event;
1441 x86_pmu.handle_irq = amd_pmu_v2_handle_irq;
1442 static_call_update(amd_pmu_test_overflow,
amd_pmu_test_overflow_status);
1443 }
That's why legacy 4-PMC PMU is probed after we disable perfctr-core.
- (boot_cpu_data.x86 < 6): No PMU.
- Without perfctr-core: 4 PMCs
- With perfctr-core: 6 PMCs
- PerfMonV2: PMCs (currently 6) + global PMU registers
May this resolve your concern in another thread that "This looks like a KVM
bug."? This isn't a KVM bug. It is because AMD's lack of the configuration
to disable PMU.
Thank you very much!
Dongli Zhang
>
>> However, the guest CPUID indicates the PerfMonV2 is still available.
>>
>> CPU:
>> Extended Performance Monitoring and Debugging (0x80000022):
>> AMD performance monitoring V2 = true
>> AMD LBR V2 = false
>> AMD LBR stack & PMC freezing = false
>> number of core perf ctrs = 0x6 (6)
>> number of LBR stack entries = 0x0 (0)
>> number of avail Northbridge perf ctrs = 0x0 (0)
>> number of available UMC PMCs = 0x0 (0)
>> active UMCs bitmask = 0x0
>>
>> Disable PerfMonV2 in CPUID when PERFCORE is disabled.
>>
>> Suggested-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
>
> Though I have above confusion of the description, the change itself looks
> good to me. So
>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
>
>> Fixes: 209b0ac12074 ("target/i386: Add PerfMonV2 feature bit")
>> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>> ---
>> Changed since v1:
>> - Use feature_dependencies (suggested by Zhao Liu).
>>
>> target/i386/cpu.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 72ab147e85..b6d6167910 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -1805,6 +1805,10 @@ static FeatureDep feature_dependencies[] = {
>> .from = { FEAT_7_1_EDX, CPUID_7_1_EDX_AVX10 },
>> .to = { FEAT_24_0_EBX, ~0ull },
>> },
>> + {
>> + .from = { FEAT_8000_0001_ECX, CPUID_EXT3_PERFCORE },
>> + .to = { FEAT_8000_0022_EAX,
>> CPUID_8000_0022_EAX_PERFMON_V2 },
>> + },
>> };
>> typedef struct X86RegisterInfo32 {
>
