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

[Sandipan Das <sandipan.das@amd.com>]

On 11/13/2022 4:03 AM, Liang Yan wrote:
> 
> On 10/31/22 10:28, Sandipan Das wrote:
>> 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@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@digtalocean.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.
> 
> 
> Hi Sandipan,
> 
> Thanks for the classification.
> Do these legacy counters belong to the AMD PMU property from a VM perspective? I mean, if I want to disable PMU for an AMD vcpu for some reason, is it possible to disable perfctr_core and the four counters, or is this not logical since the four counters could not be disabled from the bare-metal level?
> I asked because I saw 'pmu' could be disabled for Intel and ARM, but it seems not for AMD.
> 

From what I see, the four legacy counters are not tied to any processor
properties (e.g. CPUID bits). Disabling "perfctr-core" only brings the
number of supported core counters down to 4 from 6. So guests exhibit
the same behaviour as bare-metal where the legacy counters are used if
CPUID 0x80000001[ECX].PerfCtrExtCore is not set.

The "pmu" property only overrides guest CPUID. Hence it is not possible
to prevent the discovery of the legacy counters using that.

> Also, could you please list the four legacy counters here?
> 

The MSRs for the four legacy counters are:
  0xc001000[0..3] known as PERF_LEGACY_CTL[0..3], alias of PERF_CTL[0..3]
  0xc001000[4..7] known as PERF_LEGACY_CTR[0..3], alias of PERF_CTR[0..3]

You can find more details in the Processor Programming Reference (PPR) that
is appropriate for the AMD processor that you are using. PPRs can be found
at: https://bugzilla.kernel.org/show_bug.cgi?id=206537

> 
> 
>> 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
>>