Hi Liang, Peter,
On 10/31/2022 6:29 PM, Peter Zijlstra wrote:
On Thu, Oct 27, 2022 at 09:35:11AM -0400, Liang Yan wrote:There are four legacy counters that are always available even when PERFCTR_CORE
After disabling cpu.perfctr_core in qemu, I noticed that the guest kernelLooks about right, Ravi?
still loads the pmu driver while the cpuid does not have perfctl_core.
The test is running on an EPYC Rome machine.
root@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~# lscpu | grep perfctl
root@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~#
root@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~# dmesg | grep PMU
[ 0.732097] Performance Events: AMD PMU driver.
By further looking,
==> init_hw_perf_events
==> amd_pmu_init
==> amd_core_pmu_init
==>
if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
return 0;
With returning 0, it will bypass amd_pmu_init and return 0 to
init_hw_perf_events, and continue the initialization.
I am not a perf expert and not sure if it is expected for AMD PMU,
otherwise, it would be nice to return -ENODEV instead.
New output after the change:
root@ubuntu-s-4vcpu-8gb-amd-nyc1-01:~# dmesg | grep PMU
[ 0.531609] Performance Events: no PMU driver, software events only.
Signed-off-by: Liang Yan <lyan@xxxxxxxxxxxxxxx>
---
arch/x86/events/amd/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 8b70237c33f7..34d3d2944020 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -1335,7 +1335,7 @@ static int __init amd_core_pmu_init(void)
int i;
if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
- return 0;
+ return -ENODEV;
is absent. This is why the code returns 0 here. I found this to be a bit confusing
as well during PerfMonV2 development so I wrote the following patch but forgot to
send it out.
diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 262e39a85031..d3eb7b2f4dda 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -1345,6 +1345,14 @@ static int __init amd_core_pmu_init(void)
u64 even_ctr_mask = 0ULL;
int i;
+ /*
+ * All processors support four PMCs even when X86_FEATURE_PERFCTR_CORE
+ * is unavailable. They are programmable via the PERF_LEGACY_CTLx and
+ * PERF_LEGACY_CTRx registers which have the same address as that of
+ * MSR_K7_EVNTSELx and MSR_K7_PERFCTRx. For Family 17h+, these are
+ * legacy aliases of PERF_CTLx and PERF_CTRx respectively. Hence, not
+ * returning -ENODEV here.
+ */
if (!boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
return 0;
If this looks good to you, I will post it.
- Sandipan