linux-kernel - Re: [PATCH v1 09/20] x86/resctrl: Abstract and use supports_mba

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Date:   Wed, 1 Sep 2021 14:27:26 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     James Morse <james.morse@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        <shameerali.kolothum.thodi@...wei.com>,
        Jamie Iles <jamie@...iainc.com>,
        "D Scott Phillips OS" <scott@...amperecomputing.com>,
        <lcherian@...vell.com>, <bobo.shaobowang@...wei.com>
Subject: Re: [PATCH v1 09/20] x86/resctrl: Abstract and use
 supports_mba_mbps()

Hi James,

On 7/29/2021 3:35 PM, James Morse wrote:
> To determine whether the mba_mbps option to resctrl should be supported,

mba_mbps -> mba_MBps

> resctrl tests the boot cpus' x86_vendor.

CPU

> 
> This isn't portable, and needs abstracting behind a helper so this check
> can be part of the filesystem code that moves to /fs/.
> 
> Re-use the tests set_mba_sc() does to determine if the mba_sc is supported
> on this system. An 'alloc_capable' test is added so that this property
> isn't implied by 'linear'.

Why can linear not imply alloc_capable? It is a property of a MBA 
resource so if it is set then it has to be a MBA resource.

> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 19 ++++++++++++++-----
>   1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index e321ea5de562..4388685548a0 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1888,17 +1888,26 @@ void rdt_domain_reconfigure_cdp(struct rdt_resource *r)
>   }
>   
>   /*
> - * Enable or disable the MBA software controller
> - * which helps user specify bandwidth in MBps.
>    * MBA software controller is supported only if
>    * MBM is supported and MBA is in linear scale.
>    */
> +static bool supports_mba_mbps(void)
> +{
> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
> +
> +	return (is_mbm_enabled() &&
> +		r->alloc_capable && is_mba_linear());

As mentioned above I do not see need for the alloc_capable check since 
this is hardcoded to be a MBA resource for which delay_linear can only 
be set if alloc_capable is set.

> +}
> +
> +/*
> + * Enable or disable the MBA software controller
> + * which helps user specify bandwidth in MBps.
> + */
>   static int set_mba_sc(bool mba_sc)
>   {
>   	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
>   
> -	if (!is_mbm_enabled() || !is_mba_linear() ||
> -	    mba_sc == is_mba_sc(r))
> +	if (!supports_mba_mbps() || mba_sc == is_mba_sc(r))
>   		return -EINVAL;
>   
>   	r->membw.mba_sc = mba_sc;
> @@ -2317,7 +2326,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>   		ctx->enable_cdpl2 = true;
>   		return 0;
>   	case Opt_mba_mbps:
> -		if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)

I am not familiar with the history but it looks like AMD does not 
support linear (based on __rdt_get_mem_config_amd()) which may be the 
reason for this test and thus moving it to a feature check is ok.

> +		if (supports_mba_mbps())
>   			return -EINVAL;
>   		ctx->enable_mba_mbps = true;
>   		return 0;
> 

Reinette