linux-kernel - Re: [PATCH] arch/x86/events/amd/core.c: Return -ENODEV when CPU does not have PERFCTL

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Date:   Mon, 31 Oct 2022 19:58:32 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Liang Yan <lyan@...italocean.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/x86/events/amd/core.c: Return -ENODEV when CPU does
 not have PERFCTL_CORE bit

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:
>> After disabling cpu.perfctr_core in qemu, I noticed that the guest kernel
>> still loads the pmu driver while the cpuid does not have perfctl_core.
>>
>> The test is running on an EPYC Rome machine.
>> root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~# lscpu | grep perfctl
>> root@...ntu-s-4vcpu-8gb-amd-nyc1-01:~#
>> root@...ntu-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@...ntu-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@...talocean.com>
> 
> Looks about right, Ravi?
> 
>> ---
>>  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;
>>  

There are four legacy counters that are always available even when PERFCTR_CORE
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