Re: [PATCH v6 8/9] x86/resctrl: Introduce erdt_cpu_has() and erdt_support()

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Reinette,

On 8/20/2026 7:08 AM, Reinette Chatre wrote:
> Hi Chenyu,
> 
> On 7/25/26 2:23 AM, Chen Yu wrote:
>> Introduce erdt_cpu_has() to verify if a specific RDT feature is backed
>> by an ERDT table. erdt_cpu_has() is derived from rdt_cpu_has(), which
>> not only considers firmware (ERDT table and its sub-tables) support for
>> an event, but also considers userspace input like "rdt=!cmt".
>> Similar to rdt_cpu_has(), erdt_cpu_has() expects an x86 feature flag
>> as parameter.
>>
>> Introduce erdt_support() as a helper called by erdt_cpu_has() to check
>> whether the ERDT firmware supports a given feature.
>>
>> erdt_support() deliberately claims no feature yet and returns false. A
>> feature may only be reported as ERDT backed once the low level accessors
>> that read its monitoring data via MMIO exist, otherwise callers would
> 
> Are these helpers limited to monitoring ERDT features? First paragraph mentions
> "support for an event" and above mentions "read its monitoring data" that
> imply these helpers are limited to monitoring features but I do not think this is
> the case?
> 

They are not only for monitoring features, but for monitor/allocation 
features.
Let me adjust the description.

>> take the ERDT path and find no implementation behind it. Features are
>> added to erdt_support() as their MMIO implementations are introduced.
> 
> nit: "are added" -> "should be added"
> 

OK.

>>
>> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
>> Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
>> ---
>> v5->v6:
>>      New patch. Split out of the v5 "x86/resctrl: Introduce helpers to
>>      read L3 occupancy via MMIO" patch, which carried two logical
>>      changes. (Reinette Chatre)
>>      Rename erdt_support_features() to erdt_support(). (Reinette Chatre)
>>      Reword "expects x86 feature flag as parameters" to "expects an x86
>>      feature flag as parameter". (Reinette Chatre)
>>      Explain in the changelog why erdt_support() returns false here: a
>>      feature is only reported as ERDT backed once the MMIO accessors that
>>      read its monitoring data exist. (Reinette Chatre)
>>      Drop the unused asm/cpufeatures.h include. (Reinette Chatre)
>> ---
>>   arch/x86/include/asm/resctrl.h         |  1 +
>>   arch/x86/kernel/cpu/resctrl/core.c     | 23 +++++++++++++++++++++++
>>   arch/x86/kernel/cpu/resctrl/erdt.c     |  5 +++++
>>   arch/x86/kernel/cpu/resctrl/internal.h |  2 ++
>>   4 files changed, 31 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
>> index e60c2aea7ebd..5491853113dd 100644
>> --- a/arch/x86/include/asm/resctrl.h
>> +++ b/arch/x86/include/asm/resctrl.h
>> @@ -49,6 +49,7 @@ DECLARE_STATIC_KEY_FALSE(rdt_enable_key);
>>   DECLARE_STATIC_KEY_FALSE(rdt_alloc_enable_key);
>>   DECLARE_STATIC_KEY_FALSE(rdt_mon_enable_key);
>>   
>> +bool erdt_cpu_has(int flag);
>>   int erdt_get_scale(void);
>>   
> 
> Can this declaration be dropped? Below there is a duplicate declaration
> added to arch/x86/kernel/cpu/resctrl/internal.h that should be sufficient?
> 

This declaration is needed by [PATCH 9/9] in 
resctrl_arch_round_mon_val(), which
uses erdt_cpu_has(). Maybe we can drop erdt_cpu_has() from internal.h, 
although
I thought exposing both erdt_cpu_has() and erdt_support() might be 
useful too.

thanks,
Chenyu