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

[Liang Yan <lyan@digitalocean.com>]

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.

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

Thanks,
Liang


> 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
>