linux-kernel - Re: [PATCH v13 01/27] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <505f530c-810a-41a4-b3cf-7eb326bb6990@intel.com>
Date: Thu, 22 May 2025 13:51:29 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <james.morse@....com>, <dave.martin@....com>, <fenghuay@...dia.com>,
	<x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<akpm@...ux-foundation.org>, <thuth@...hat.com>, <rostedt@...dmis.org>,
	<ardb@...nel.org>, <gregkh@...uxfoundation.org>,
	<daniel.sneddon@...ux.intel.com>, <jpoimboe@...nel.org>,
	<alexandre.chartre@...cle.com>, <pawan.kumar.gupta@...ux.intel.com>,
	<thomas.lendacky@....com>, <perry.yuan@....com>, <seanjc@...gle.com>,
	<kai.huang@...el.com>, <xiaoyao.li@...el.com>, <kan.liang@...ux.intel.com>,
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	<mario.limonciello@....com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <peternewman@...gle.com>,
	<maciej.wieczor-retman@...el.com>, <eranian@...gle.com>,
	<Xiaojian.Du@....com>, <gautham.shenoy@....com>
Subject: Re: [PATCH v13 01/27] x86/cpufeatures: Add support for Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Babu,

On 5/15/25 3:51 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, event pair 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.
> 
> 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 nee
> dto unassign one or more active counters to free up countes for new

"nee dto" -> "need to"
"countes" -> "counters"


> assignments. The interface will provide options to assign or unassign

"The interface will" -> "resctrl will"?

> events through the group-specific interface file.
> 
> The feature can be detected via CPUID_Fn80000020_EBX_x00 bit 5.

"The feature can be detected" -> "The feature is detected"

> 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>
> ---

...
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  arch/x86/kernel/cpu/cpuid-deps.c   | 2 ++
>  arch/x86/kernel/cpu/scattered.c    | 1 +
>  3 files changed, 4 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 6c2c152d8a67..d5c14dc678df 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -481,6 +481,7 @@
>  #define X86_FEATURE_AMD_HETEROGENEOUS_CORES (21*32 + 6) /* Heterogeneous Core Topology */
>  #define X86_FEATURE_AMD_WORKLOAD_CLASS	(21*32 + 7) /* Workload Classification */
>  #define X86_FEATURE_PREFER_YMM		(21*32 + 8) /* Avoid ZMM registers due to downclocking */
> +#define X86_FEATURE_ABMC		(21*32 + 9) /* Assignable Bandwidth Monitoring Counters */
>  
>  /*
>   * BUG word(s)
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index a2fbea0be535..2f54831e04e5 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -71,6 +71,8 @@ static const struct cpuid_dep cpuid_deps[] = {
>  	{ X86_FEATURE_CQM_MBM_LOCAL,		X86_FEATURE_CQM_LLC   },
>  	{ X86_FEATURE_BMEC,			X86_FEATURE_CQM_MBM_TOTAL   },
>  	{ X86_FEATURE_BMEC,			X86_FEATURE_CQM_MBM_LOCAL   },
> +	{ X86_FEATURE_ABMC,			X86_FEATURE_CQM_MBM_TOTAL   },
> +	{ X86_FEATURE_ABMC,			X86_FEATURE_CQM_MBM_LOCAL   },

Is this dependency still accurate now that the implementation switched to the 
"extended event ID" variant of ABMC that no longer uses the event IDs associated
with X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL?

>  	{ X86_FEATURE_AVX512_BF16,		X86_FEATURE_AVX512VL  },
>  	{ X86_FEATURE_AVX512_FP16,		X86_FEATURE_AVX512BW  },
>  	{ X86_FEATURE_ENQCMD,			X86_FEATURE_XSAVES    },
> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
> index 16f3ca30626a..3b72b72270f1 100644
> --- a/arch/x86/kernel/cpu/scattered.c
> +++ b/arch/x86/kernel/cpu/scattered.c
> @@ -49,6 +49,7 @@ static const struct cpuid_bit cpuid_bits[] = {
>  	{ X86_FEATURE_MBA,			CPUID_EBX,  6, 0x80000008, 0 },
>  	{ X86_FEATURE_SMBA,			CPUID_EBX,  2, 0x80000020, 0 },
>  	{ X86_FEATURE_BMEC,			CPUID_EBX,  3, 0x80000020, 0 },
> +	{ X86_FEATURE_ABMC,			CPUID_EBX,  5, 0x80000020, 0 },
>  	{ X86_FEATURE_AMD_WORKLOAD_CLASS,	CPUID_EAX, 22, 0x80000021, 0 },
>  	{ X86_FEATURE_PERFMON_V2,		CPUID_EAX,  0, 0x80000022, 0 },
>  	{ X86_FEATURE_AMD_LBR_V2,		CPUID_EAX,  1, 0x80000022, 0 },

Reinette