linux-kernel - Re: [PATCH v16 08/34] x86,fs/resctrl: Detect Assignable Bandwidth Monitoring feature details

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Message-ID: <aed0a452-9b16-46ca-8075-7be9eaa0cfd1@intel.com>
Date: Wed, 30 Jul 2025 12:49:56 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<james.morse@....com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>
CC: <Dave.Martin@....com>, <x86@...nel.org>, <hpa@...or.com>,
	<akpm@...ux-foundation.org>, <paulmck@...nel.org>, <rostedt@...dmis.org>,
	<Neeraj.Upadhyay@....com>, <david@...hat.com>, <arnd@...db.de>,
	<fvdl@...gle.com>, <seanjc@...gle.com>, <jpoimboe@...nel.org>,
	<pawan.kumar.gupta@...ux.intel.com>, <xin@...or.com>,
	<manali.shukla@....com>, <tao1.su@...ux.intel.com>, <sohil.mehta@...el.com>,
	<kai.huang@...el.com>, <xiaoyao.li@...el.com>, <peterz@...radead.org>,
	<xin3.li@...el.com>, <kan.liang@...ux.intel.com>,
	<mario.limonciello@....com>, <thomas.lendacky@....com>, <perry.yuan@....com>,
	<gautham.shenoy@....com>, <chang.seok.bae@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peternewman@...gle.com>, <eranian@...gle.com>
Subject: Re: [PATCH v16 08/34] x86,fs/resctrl: Detect Assignable Bandwidth
 Monitoring feature details

Hi Babu,

On 7/25/25 11:29 AM, Babu Moger wrote:
> ABMC feature details are reported via CPUID Fn8000_0020_EBX_x5.
> Bits Description
> 15:0 MAX_ABMC Maximum Supported Assignable Bandwidth
>      Monitoring Counter ID + 1
> 
> 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).
> 
> Detect the feature and number of assignable counters supported. For
> backward compatibility, upon detecting the assignable counter feature,
> enable the mbm_total_bytes and mbm_local_bytes events that users are
> familiar with as part of original L3 MBM support.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---

...

> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 267e9206a999..09cb5a70b1cb 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -883,6 +883,8 @@ static __init bool get_rdt_mon_resources(void)
>  		resctrl_enable_mon_event(QOS_L3_MBM_LOCAL_EVENT_ID);
>  		ret = true;
>  	}
> +	if (rdt_cpu_has(X86_FEATURE_ABMC))
> +		ret = true;
>  
>  	if (!ret)
>  		return false;
> @@ -990,7 +992,8 @@ void resctrl_cpu_detect(struct cpuinfo_x86 *c)
>  

To complement the change below, shouldn't the snippet that precedes it look like:
	if (!cpu_has(c, X86_FEATURE_CQM_LLC) && !cpu_has(c, X86_FEATURE_ABMC)) {
		...
		return;
	}

>  	if (cpu_has(c, X86_FEATURE_CQM_OCCUP_LLC) ||
>  	    cpu_has(c, X86_FEATURE_CQM_MBM_TOTAL) ||
> -	    cpu_has(c, X86_FEATURE_CQM_MBM_LOCAL)) {
> +	    cpu_has(c, X86_FEATURE_CQM_MBM_LOCAL) ||
> +	    cpu_has(c, X86_FEATURE_ABMC)) {
>  		u32 eax, ebx, ecx, edx;
>  
>  		/* QoS sub-leaf, EAX=0Fh, ECX=1 */
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 2558b1bdef8b..0a695ce68f46 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -339,6 +339,7 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>  	unsigned int mbm_offset = boot_cpu_data.x86_cache_mbm_width_offset;
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>  	unsigned int threshold;
> +	u32 eax, ebx, ecx, edx;
>  
>  	snc_nodes_per_l3_cache = snc_get_config();
>  
> @@ -368,14 +369,18 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>  	 */
>  	resctrl_rmid_realloc_threshold = resctrl_arch_round_mon_val(threshold);
>  
> -	if (rdt_cpu_has(X86_FEATURE_BMEC)) {
> -		u32 eax, ebx, ecx, edx;
> -
> +	if (rdt_cpu_has(X86_FEATURE_BMEC) || rdt_cpu_has(X86_FEATURE_ABMC)) {
>  		/* Detect list of bandwidth sources that can be tracked */
>  		cpuid_count(0x80000020, 3, &eax, &ebx, &ecx, &edx);
>  		r->mon.mbm_cfg_mask = ecx & MAX_EVT_CONFIG_BITS;

I interpret this mbm_cfg_mask initialization that an ABMC system will report which of
the memory transactions can be monitored. 
In patch #15 "fs/resctrl: Introduce event configuration field in struct mon_evt"
the event configurations of memory transactions that should be monitored are hardcoded
as below without taking into account what the system supports:

	resctrl_mon_resource_init() {
		...
		mon_event_all[QOS_L3_MBM_TOTAL_EVENT_ID].evt_cfg = MAX_EVT_CONFIG_BITS;
		mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID].evt_cfg = READS_TO_LOCAL_MEM |
								   READS_TO_LOCAL_S_MEM |
								   NON_TEMP_WRITE_TO_LOCAL_MEM;
		...
	}

It may thus be that a system may not support all memory transactions it is configured to
monitor. It seems to me that the initialization done in resctrl_mon_resource_init() needs
to take r->mon.mbm_cfg_mask (what the system supports) into account? If so, then
the same hardcoding done by patch #32 in resctrl_mbm_assign_mode_write() should
also be changed.
	
Reinette