linux-kernel - Re: [PATCH v3 06/10] x86/resctrl: Introduce mon_configurable to detect Bandwidth Monitoring Event Configuration

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Date:   Wed, 24 Aug 2022 14:15:33 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Babu Moger <babu.moger@....com>, <fenghua.yu@...el.com>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>
CC:     <eranian@...gle.com>, <dave.hansen@...ux.intel.com>,
        <x86@...nel.org>, <hpa@...or.com>, <corbet@....net>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <bagasdotme@...il.com>
Subject: Re: [PATCH v3 06/10] x86/resctrl: Introduce mon_configurable to
 detect Bandwidth Monitoring Event Configuration

Hi Babu,

On 8/22/2022 6:43 AM, Babu Moger wrote:
> Newer AMD processors support the new feature Bandwidth Monitoring Event
> Configuration (BMEC). The events mbm_total_bytes and mbm_local_bytes
> are configurable when this feature is present.
> 
> Set mon_configurable if the feature is available.
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> Reviewed-by: Ingo Molnar <mingo@...nel.org>
> ---
>  arch/x86/kernel/cpu/resctrl/monitor.c  |   14 ++++++++++++++
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c |   17 +++++++++++++++++
>  include/linux/resctrl.h                |    1 +
>  3 files changed, 32 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index eaf25a234ff5..b9de417dac1c 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -682,6 +682,16 @@ static void l3_mon_evt_init(struct rdt_resource *r)
>  		list_add_tail(&mbm_local_event.list, &r->evt_list);
>  }
>  
> +
> +void __rdt_get_mon_l3_config_amd(struct rdt_resource *r)
> +{
> +	/*
> +	 * Check if CPU supports the Bandwidth Monitoring Event Configuration
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_BMEC))
> +		r->mon_configurable = true;
> +}

Could this rather use rdt_cpu_has() with the added support for disabling
the feature via kernel parameter?


> +
>  int rdt_get_mon_l3_config(struct rdt_resource *r)
>  {
>  	unsigned int mbm_offset = boot_cpu_data.x86_cache_mbm_width_offset;
> @@ -714,6 +724,10 @@ int rdt_get_mon_l3_config(struct rdt_resource *r)
>  	if (ret)
>  		return ret;
>  
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> +		__rdt_get_mon_l3_config_amd(r);
> +
> +

Why is this vendor check needed? Is X86_FEATURE_BMEC not sufficient?

>  	l3_mon_evt_init(r);
>  
>  	r->mon_capable = true;
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index fc5286067201..855483b297a8 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -995,6 +995,16 @@ static int rdt_num_rmids_show(struct kernfs_open_file *of,
>  	return 0;
>  }
>  
> +static int rdt_mon_configurable_show(struct kernfs_open_file *of,
> +				     struct seq_file *seq, void *v)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +
> +	seq_printf(seq, "%d\n", r->mon_configurable);

Why is this file needed? It seems that the next patches also introduce
files in support of this new feature that will make the actual configuration
data accessible - those files are only created if this feature is supported.
Would those files not be sufficient for user space to learn about the feature
support?

Reinette