linux-kernel - Re: [PATCH v13 04/27] x86/resctrl: Detect Assignable Bandwidth Monitoring feature details

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Message-ID: <bedd1ddc-d1de-48bb-926d-074fe6f6c253@intel.com>
Date: Thu, 22 May 2025 13:54:19 -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>,
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	<Xiaojian.Du@....com>, <gautham.shenoy@....com>
Subject: Re: [PATCH v13 04/27] x86/resctrl: Detect Assignable Bandwidth
 Monitoring feature details

Hi Babu,

On 5/15/25 3:51 PM, 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 monitoring counters supported.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---

...

> ---
>  arch/x86/kernel/cpu/resctrl/monitor.c | 9 +++++++--
>  include/linux/resctrl.h               | 4 ++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index aeb2a9283069..fd2761d9f3f7 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -345,6 +345,7 @@ 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();
>  
> @@ -375,13 +376,17 @@ 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;
> -
>  		/* 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)) {
> +		r->mon.mbm_cntr_assignable = true;
> +		cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
> +		r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
> +	}
> +

Shouldn't ABMC detection also include enumeration of which configurations
are supported? From what I can tell, looking ahead patch #18 hardcodes definitions
of all known "bandwidth types" (which term to use TBD) and then patch #20 allows
*any* of these types to be configured irrespective of whether system
supports it.
AMD spec mentions "The types of L3 transactions that ABMC can track are
configurable and identified by CPUID Fn8000_0020_ECX_x3."  It thus looks
like the enumeration of r->mon.mbm_cfg_mask when BMEC is enabled is
required for ABMC also and used by this implementation.

>  	r->mon_capable = true;
>  
>  	return 0;
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 2a8fa454d3e6..065fb6e38933 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -260,11 +260,15 @@ enum resctrl_schema_fmt {
>   * @num_rmid:		Number of RMIDs available
>   * @mbm_cfg_mask:	Bandwidth sources that can be tracked when bandwidth
>   *			monitoring events can be configured.
> + * @num_mbm_cntrs:	Number of assignable monitoring counters
> + * @mbm_cntr_assignable:Is system capable of supporting monitor assignment?

"monitor assignment" has not been used so far, was this intended to be
"counter assignment"?

Reinette