Re: [PATCH v4 6/6] x86/resctrl: Add support for L3 occupancy monitoring via RMID MMIO read

[Reinette Chatre]

Hi Chenyu,

On 6/22/26 7:09 AM, Chen, Yu C wrote:
> On 6/19/2026 7:40 AM, Reinette Chatre wrote:
>> On 6/13/26 12:57 AM, Chen Yu wrote:

...

>>> diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
>>> index 97c2f6bc7a5f..9b3b03279dd8 100644
>>> --- a/arch/x86/include/asm/resctrl.h
>>> +++ b/arch/x86/include/asm/resctrl.h
>>> @@ -41,6 +41,8 @@ struct resctrl_pqr_state {
>>>   };
>>>     bool erdt_enabled(void);
>>> +struct rdt_domain_hdr;
>>> +int erdt_mon_read(struct rdt_domain_hdr *hdr, int ev_id, int rmid, u64 *val);
>>>     DECLARE_PER_CPU(struct resctrl_pqr_state, pqr_state);
>>>   diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
>>> index 90730f0851fa..fe812f7190fc 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>>> @@ -965,7 +965,7 @@ static __init bool get_rdt_mon_resources(void)
>>>       bool ret = false;
>>>         if (rdt_cpu_has(X86_FEATURE_CQM_OCCUP_LLC)) {
>>> -        resctrl_enable_mon_event(QOS_L3_OCCUP_EVENT_ID, false, 0, NULL);
>>> +        resctrl_enable_mon_event(QOS_L3_OCCUP_EVENT_ID, erdt_enabled(), 0, NULL);
>>>           ret = true;
>>>       }
>>>       if (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL)) {
>>
>> As mentioned in patch #1, when erdt_enabled() is true the enumeration still proceeds to
>> enumerate the monitoring properties via CPUID to discover the number of RMIDs that the
>> *MSR* supports and use it as the maximum RMID (and thus the maximum number of registers)
>> that MMIO supports?
>>
> 
> OK, will switch to the maximum RMID exposed by ACPI table, if erdt_enabled() is true.

I believe the issue is larger than just the RMID enumeration. The CPUID and ACPI enumeration
appears to be fully intertwined. Taking a closer look at what above code does:
it checks *CPUID* whether CMT is enabled and then enables the LLC occupancy event to blindly use
MMIO if ERDT is enabled, irrespective of whether the ERDT tables include a cache monitoring table
or not. How is it guaranteed that if ERDT is enabled that there is a cache monitoring table?
Should it not be the existence of the ACPI cache monitoring table and its properties that
determines whether the LLC occupancy counter using MMIO registers should be enabled?

Reinette