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

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Message-ID: <11949172-e319-4d2e-b977-67aedeac5c1b@intel.com>
Date: Thu, 4 Sep 2025 10:35:48 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <Dave.Martin@....com>, <james.morse@....com>, <x86@...nel.org>,
	<hpa@...or.com>, <akpm@...ux-foundation.org>, <rostedt@...dmis.org>,
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Subject: Re: [PATCH v17 05/33] x86/cpufeatures: Add support for Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Babu,

On 9/4/25 10:21 AM, Moger, Babu wrote:
> 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@....com>
>> Reviewed-by: Reinette Chatre <reinette.chatre@...el.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.

Yes, thank you. I think this is a helpful addition. I assume this change
will be applied to all patches in this series that contain this pattern?

Reinette