Re: [PATCH v17 05/33] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

["Moger, Babu" <babu.moger@amd.com>]

Hi Reinette,

On 8/14/25 21:25, 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 during a particular time. Users do not have the option to
> monitor a group or set of groups for a certain period of time without
> worrying about RMID being reset in between.
> 
> The ABMC feature allows users to assign a hardware counter to an RMID,
> event pair and monitor bandwidth usage as long as it is assigned. The
> hardware continues to track the assigned counter until it is explicitly
> unassigned by the user. There is no need to worry about counters being
> reset during this period. Additionally, the user can specify the type of
> memory transactions (e.g., reads, writes) for the counter to track.
> 
> Without ABMC enabled, monitoring will work in current mode without
> assignment option.
> 
> The Linux resctrl subsystem provides an interface that allows monitoring of
> up to two memory bandwidth events per group, selected from a combination of
> available total and local events. When ABMC is enabled, two events will be
> assigned to each group by default, in line with the current interface
> design. Users will also have the option to configure which types of memory
> transactions are counted by these events.
> 
> Due to the limited number of available counters (32), users may quickly
> exhaust the available counters. If the system runs out of assignable ABMC
> counters, the kernel will report an error. In such cases, users will need
> to unassign one or more active counters to free up counters for new
> assignments. resctrl will provide options to assign or unassign events
> through the group-specific interface file.
> 
> The feature is 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).
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

This is an FYI. I am updating the text about the documentation to address
your comment.
https://lore.kernel.org/lkml/53387aa7-0e72-4bf9-a188-43c0f78c2d6f@intel.com/


The ABMC feature details are documented in APM [1] available from [2].
[1] AMD64 Architecture Programmer's Manual Volume 2: System Programming
   Publication # 24593 Revision 3.41 section 19.3.3.3 Assignable Bandwidth
   Monitoring (ABMC).

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2]



Hope it is fine.

Thanks

Babu