Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Babu,

On 7/28/26 1:43 PM, Babu Moger wrote:
> Hi Tony,
> 
> On 7/28/26 15:24, Luck, Tony wrote:
>>>> 1) Does it start over from zero?
>>>>      Seems a sane option.
>>>> 2) Or does the monitor group see whatever the counter was at
>>>> when being used by some other group?
>>>>      Seems less sane.
>>>
>>> As per this code, it should start over from 0.
>>>
>>>    } else if (ret == -EINVAL) {
>>>                   am = get_arch_mbm_state(hw_dom, rmid, eventid);
>>>                   if (am)
>>>                           am->prev_msr = 0;
>>>           }
>>
>> Nope. If you want the value reported to the user to start over from zero,
>> then prev_msr should be set to the current value of the counter. That
>> would make the next read start from zero.
>>
> When an RMID is reallocated, the hardware detects the state
> transition and the first read returns "Unavailable". At that point,
> the counter is reset and begins counting again from 0. This is the
> expected behavior on AMD systems. Therefore, setting prev_msr to 0
> is the correct behavior.

I believe Tony was curious how this looks from the user's perspective.

From what I understand it is neither of the presented options but instead:

3) Internal cache of hardware counter (prev_msr) is reset when resctrl discovers
   that the underlying counter has been re-assigned.
   There is a separate software maintained cumulative count to which the
   hardware counter value is added as the event is tracked. This cumulative
   count is the value returned to the user.
   The user thus always sees that the event count increase and it is
   guaranteed to only increase based on a portion of memory bandwidth
   consumed by the RMID/event being tracked.
   The returned (cumulative) count will not be accurate if the underlying
   counter was re-assigned. The user does not have insight into how long the
   underlying counter(s) was assigned to the event being counted.

Reinette