Re: [PATCH v5 01/20] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

[Moger, Babu]

Hi Reinette,

On 7/12/24 16:55, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/3/24 2:48 PM, Babu Moger wrote:
>> Users can create as many monitor groups as RMIDs supported by the hardware.
>> However, bandwidth monitoring feature on AMD system only guarantees that
>> RMIDs currently assigned to a processor will be tracked by hardware. The
>> counters of any other RMIDs which are no longer being tracked will be
>> reset to zero. The MBM event counters return "Unavailable" for the RMIDs
>> that are not tracked by hardware. So, there can be only limited number of
>> groups that can give guaranteed monitoring numbers. With ever changing
>> configurations there is no way to definitely know which of these groups
>> are being tracked for certain point of time. Users do not have the option
>> to monitor a group or set of groups for certain period of time without
>> worrying about RMID being reset in between.
>>
>> The ABMC feature provides an option to the user to assign a hardware
>> counter to an RMID and monitor the bandwidth as long as it is assigned.
>> The assigned RMID will be tracked by the hardware until the user unassigns
>> it manually. There is no need to worry about counters being reset during
>> this period. Additionally, the user can specify a bitmask identifying the
>> specific bandwidth types from the given source to track with the counter.
>>
>> Without ABMC enabled, monitoring will work in current mode without
>> assignment option.
>>
>> Linux resctrl subsystem provides the interface to count maximum of two
>> memory bandwidth events per group, from a combination of available total
>> and local events. Keeping the current interface, users can enable a maximum
>> of 2 ABMC counters per group. User will also have the option to enable only
>> one counter to the group. If the system runs out of assignable ABMC
>> counters, kernel will display an error. Users need to disable an already
>> enabled counter to make space for new assignments.
>>
>> The feature can be detected via CPUID_Fn80000020_EBX_x00 bit 5.
>> Bits Description
>> 5    ABMC (Assignable Bandwidth Monitoring Counters)
>>
>> The feature details are documented in APM listed below [1].
>> [1] AMD64 Architecture Programmer's Manual Volume 2: System Programming
>> Publication # 24593 Revision 3.41 section 19.3.3.3 Assignable Bandwidth
>> Monitoring (ABMC).
>>
>> Note: Checkpatch checks/warnings are ignored to maintain coding style.
> 
> This note may be more appropriate below the '---' separator line.

Sure.

> 
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
>> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
>> ---
> 
> Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>

-- 
Thanks
Babu Moger