Re: [PATCH v2 1/3] x86/resctrl: Fix ABMC counter programming for extended counter ranges

[Reinette Chatre]

Hi Babu,

On 9/14/26 2:30 PM, Reinette Chatre wrote:
> On 9/14/26 10:17 AM, Babu Moger wrote:
>> On 9/11/26 17:03, Reinette Chatre wrote:
>>> On 9/4/26 11:06 AM, Babu Moger wrote:
> 
> ...
> 
>>>>
>>>> Fix the issues with the following changes:
>>>>
>>>> 1. Update the cntr_id field handling to support the full hardware ABMC
>>>> counter range and ensure that counter programming does not interfere with
>>>> unrelated counters.
>>>
>>> Sashiko's assessment that this statement is not accurate looks correct to me.
>>>
>>> Looks like the enumeration needs a check to limit the number of supported
>>> counters if the hardware supports more than what can be configured? It seems
>>> awkward that such hardware could exist and unclear why the spec has a mismatch in
>>
>>
>> You're right. The spec overlooked this scenario. I've notified the concerned team.
>>
>>
>> Do you want me add this check?
>>
>> -               r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
>> +               r->mon.num_mbm_cntrs = (ebx & GENMASK(12, 0)) + 1;
>>
> 
> Yes please.
> 
>>
>>> the number of bits here. Even if cntr_id expands further to use the remaining
>>> reserved bits it would still not be sufficient to configure all the numbers
>>> that hardware may claim to support. Are there perhaps more field width changes
>>> in this upcoming spec update?
>>
>> Two fields(ctrl_id and bw_src) in here are changing for this register. I dont know about other changes yet.
>>
>>>
>>>>
>>>> 2. Expand the bw_src field to 15 bits.
>>>
>>> Do the comments describing the RMID field width when reading the monitoring
>>> data (__cntr_id_read() and __rmid_read_phys()) need an update also?
>>
>> The RMID field width is expected to change. We can revisit this when that happens.
> 
> 
> This patch is that change, no? bw_src is expanded to 15 bits, and bw_src contains
> the RMID.
> 
> This is the new RMID field width for monitoring configuration, after configuration
> the counter needs to be read for the monitoring data via MSR_IA32_QM_EVTSEL and per
> __cntr_id_read() the RMID field is still restricted (although not enforce by resctrl)
> to 12 bits:
> 
> 	* 43:32   RMID            RMID or counter ID in ABMC mode
> 
> resctrl also supports switching between "default" and "mbm_event" mode and there
> is no support for the number of RMIDs changing between the modes ... this reading
> an event in "default" mode should support the same number of RMIDs as 
> "mbm_event" mode and we see in __rmid_read_phys() (although, again not enforced)
> that it supports even fewer bits for RMID
> 	
> 	IA32_QM_EVTSEL.RMID (bits 41:32) are configured with valid RMID
> 
> Looks like after this patch there are quite a few different field widths for RMID
> between what is documented, used directly, and actually enforced.
> 
To add to this it looks like PLZA currently supports 12 bit RMIDs per

	https://lore.kernel.org/lkml/702b88901bc62f07829144a96cc887471b427da8.1787772750.git.babu.moger@xxxxxxx/

I assume this field will also be updated to 15 bits?

Reinette