linux-kernel - Re: [PATCH v4 03/13] x86/cpufeatures: Add Slow Memory Bandwidth Allocation feature flag

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Date:   Fri, 16 Sep 2022 08:54:22 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Babu Moger <babu.moger@....com>, <corbet@....net>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>
CC:     <fenghua.yu@...el.com>, <dave.hansen@...ux.intel.com>,
        <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
        <akpm@...ux-foundation.org>, <quic_neeraju@...cinc.com>,
        <rdunlap@...radead.org>, <damien.lemoal@...nsource.wdc.com>,
        <songmuchun@...edance.com>, <peterz@...radead.org>,
        <jpoimboe@...nel.org>, <pbonzini@...hat.com>,
        <chang.seok.bae@...el.com>, <pawan.kumar.gupta@...ux.intel.com>,
        <jmattson@...gle.com>, <daniel.sneddon@...ux.intel.com>,
        <sandipan.das@....com>, <tony.luck@...el.com>,
        <james.morse@....com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <bagasdotme@...il.com>,
        <eranian@...gle.com>
Subject: Re: [PATCH v4 03/13] x86/cpufeatures: Add Slow Memory Bandwidth
 Allocation feature flag

Hi Babu,

On 9/7/2022 11:00 AM, Babu Moger wrote:
> Adds the new AMD feature X86_FEATURE_SMBA. With this feature, the QOS

Adds -> Add

> enforcement policies can be applied to external slow memory connected
> to the host. QOS enforcement is accomplished by assigning a Class Of
> Service (COS) to a processor and specifying allocations or limits for
> that COS for each resource to be allocated.
> 
> This feature is identified by the CPUID Function 8000_0020_EBX_x0.
> 
> CPUID Fn8000_0020_EBX_x0 AMD Bandwidth Enforcement Feature Identifiers (ECX=0)
> Bits    Field Name      Description
> 2       L3SBE           L3 external slow memory bandwidth enforcement
> 
> Currently, CXL.memory is the only supported "slow" memory device. With the
> support of SMBA feature the hardware enables bandwidth allocation on the
> slow memory devices. If there are multiple slow memory devices in the system,
> then the throttling logic groups all the slow sources together and applies
> the limit on them as a whole.

The above is a useful addition (made in patch 13/13) to the documentation ...

> 
> The presence of the SMBA feature(with CXL.memory) is independent of whether
> slow memory device is actually present in the system. If there is no slow
> memory in the system, then setting a SMBA limit will have no impact on the
> performance of the system.

... could the above snippet also please be added to the documentation?

> 
> Presence of CXL memory can be identified by numactl command.
> 
> $numactl -H
> available: 2 nodes (0-1)
> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
> node 0 size: 63678 MB node 0 free: 59542 MB
> node 1 cpus:
> node 1 size: 16122 MB
> node 1 free: 15627 MB
> node distances:
> node   0   1
>    0:  10  50
>    1:  50  10
> 
> CPU list for CXL memory will be emply. The cpu-cxl node distance is greater

emply -> empty?

> than cpu-to-cpu distances. Node 1 has the CXL memory in this case. CXL
> memory can also be identified using ACPI SRAT table and memory maps.
> 
> Feature description is available in the specification, "AMD64 Technology Platform Quality
> of Service Extensions, Revision: 1.03 Publication # 56375 Revision: 1.03 Issue Date: February 2022".

Please shorten these lines to the recommended 75 chars per line.

> 
> Link: https://www.amd.com/en/support/tech-docs/amd64-technology-platform-quality-service-extensions
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
>  arch/x86/include/asm/cpufeatures.h |    1 +
>  arch/x86/kernel/cpu/scattered.c    |    1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 235dc85c91c3..1815435c9c88 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -304,6 +304,7 @@
>  #define X86_FEATURE_UNRET		(11*32+15) /* "" AMD BTB untrain return */
>  #define X86_FEATURE_USE_IBPB_FW		(11*32+16) /* "" Use IBPB during runtime firmware calls */
>  #define X86_FEATURE_RSB_VMEXIT_LITE	(11*32+17) /* "" Fill RSB on VM exit when EIBRS is enabled */
> +#define X86_FEATURE_SMBA		(11*32+18) /* SLOW Memory Bandwidth Allocation */

Why is "SLOW" in all caps? 

>  
>  /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
>  #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
> index fd44b54c90d5..885ecf46abb2 100644
> --- a/arch/x86/kernel/cpu/scattered.c
> +++ b/arch/x86/kernel/cpu/scattered.c
> @@ -44,6 +44,7 @@ static const struct cpuid_bit cpuid_bits[] = {
>  	{ X86_FEATURE_CPB,		CPUID_EDX,  9, 0x80000007, 0 },
>  	{ X86_FEATURE_PROC_FEEDBACK,    CPUID_EDX, 11, 0x80000007, 0 },
>  	{ X86_FEATURE_MBA,		CPUID_EBX,  6, 0x80000008, 0 },
> +	{ X86_FEATURE_SMBA,             CPUID_EBX,  2, 0x80000020, 0 },
>  	{ X86_FEATURE_PERFMON_V2,	CPUID_EAX,  0, 0x80000022, 0 },
>  	{ 0, 0, 0, 0, 0 }
>  };
> 
> 

Could you please follow the coding style (wrt tabs vs. spaces) of the area you
are contributing to here? Please do so in all patches in this series.

Reinette