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

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

Hi Reinette,

On 6/23/2026 5:30 AM, Reinette Chatre wrote:
> Hi Chenyu,
>>>> 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?
>

I see. How about replacing erdt_enabled() with fine-grained helper 
functions such as
erdt_has_cmrc(), erdt_has_mmrc(), and erdt_has_marc()? The latter two 
will be added
later for region-aware MBM/MBA. CMRC, MMRC and MARC are not guaranteed 
to coexist,
so splitting them into separate helpers would offer finer control.

thanks,
Chenyu