linux-kernel - Re: [PATCH v15 08/34] x86,fs/resctrl: Detect Assignable Bandwidth Monitoring feature details

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Message-ID: <ca71d550-86d1-40ac-a416-c02fbc1ed77e@intel.com>
Date: Thu, 17 Jul 2025 11:45:51 -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 v15 08/34] x86,fs/resctrl: Detect Assignable Bandwidth
 Monitoring feature details

Hi Babu,

On 7/8/25 3:17 PM, Babu Moger wrote:
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 267e9206a999..b48cc67cbbe3 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -990,7 +990,8 @@ void resctrl_cpu_detect(struct cpuinfo_x86 *c)
>  
>  	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)) {

I interpret this to mean that ABMC does not depend on any of the
"legacy" monitoring features.

>  		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)

rdt_get_mon_l3_config() is only called when one or more of the "legacy" monitoring
features are supported ...

>  	 */
>  	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;
>  	}
>  
> +	if (rdt_cpu_has(X86_FEATURE_ABMC)) {

... so having this check within rdt_get_mon_l3_config() implies that ABMC depends
on the "legacy" monitoring features. This contradicts the change to resctrl_cpu_detect()
though so it seems that get_rdt_mon_resources() is missing a check for ABMC to ensure
rdt_get_mon_l3_config() is called when ABMC is supported (but "legacy" monitoring features
are not)?

Reinette