Re: [PATCH] perf/x86/uncore: Correct the number of CHAs on EMR

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-09-03 4:40 a.m., Ingo Molnar wrote:
> 
> * kan.liang@linux.intel.com <kan.liang@linux.intel.com> wrote:
> 
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The MSR UNC_CBO_CONFIG, which was used to detect the number of CHAs on
>> SPR, is broken on EMR XCC. It always returns 0.
>>
>> Roll back to the discovery method, which can give the correct number for
>> this case.
>>
>> Fixes: 38776cc45eb7 ("perf/x86/uncore: Correct the number of CHAs on SPR")
>> Reported-by: Stephane Eranian <eranian@google.com>
>> Reported-by: Yunying Sun <yunying.sun@intel.com>
>> Tested-by: Yunying Sun <yunying.sun@intel.com>
>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
>>  arch/x86/events/intel/uncore_snbep.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/events/intel/uncore_snbep.c b/arch/x86/events/intel/uncore_snbep.c
>> index d49e90dc04a4..c41d7d46481c 100644
>> --- a/arch/x86/events/intel/uncore_snbep.c
>> +++ b/arch/x86/events/intel/uncore_snbep.c
>> @@ -6475,7 +6475,9 @@ void spr_uncore_cpu_init(void)
>>  	type = uncore_find_type_by_id(uncore_msr_uncores, UNCORE_SPR_CHA);
>>  	if (type) {
>>  		rdmsrl(SPR_MSR_UNC_CBO_CONFIG, num_cbo);
>> -		type->num_boxes = num_cbo;
>> +		/* The MSR doesn't work on the EMR XCC. Roll back to the discovery method. */
>> +		if (num_cbo)
>> +			type->num_boxes = num_cbo;
> 
> So in the zero case we don't write type->num_boxes and leave it as-is.
> 
> How does this fall back to the discovery method, is the existing (default?) 
> value of type->num_boxes some special value?
> 

Starts from SPR, the basic uncore PMON information are retrieved from
the discovery table (resides in an MMIO space populated by BIOS.). It is
called the discovery method. The existing value of the type->num_boxes
is from the discovery table.

On some SPR variants, there is a firmware bug. So the value from the
discovery table is incorrect. We use the value from
SPR_MSR_UNC_CBO_CONFIG to replace the one from the discovery table.
38776cc45eb7 ("perf/x86/uncore: Correct the number of CHAs on SPR")

Unfortunately, the SPR_MSR_UNC_CBO_CONFIG isn't available for the EMR
XCC (It works well for other EMR variants). But the above firmware bug
doesn't impact the EMR XCC. So this patch NOT lets the value from the
SPR_MSR_UNC_CBO_CONFIG replace the existing value from the discovery table.


Thanks,
Kan