linux-kernel - Re: [PATCH v3 02/10] x86/cpufeatures: Add Slow Memory Bandwidth Allocation feature flag

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Date:   Tue, 23 Aug 2022 15:47:28 -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 02/10] x86/cpufeatures: Add Slow Memory Bandwidth
 Allocation feature flag

Hi Babu,

On 8/22/2022 6:42 AM, Babu Moger wrote:
> Adds the new AMD feature X86_FEATURE_SMBA. With this feature, the QOS
> 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
> 
> 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".
> 
> 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>
> Reviewed-by: Ingo Molnar <mingo@...nel.org>
> ---

resctrl currently supports "memory bandwidth allocation" and this series adds
"slow memory bandwidth allocation". Could you please provide more detail about
what the difference is between "MBA" and "SMBA"? It is clear that the implementation
treats them as different resources, but both resources are associated with L3 cache
domains and (from what I understand) throttling always occurs at the CPU. Can both
types of memory resources thus be seen as downstream from L3 cache? How can
a user know what memory is considered when configuring MBA and what memory is
considered when configuring SMBA? Additionally, I do find the term "slow" to be
vague as a way to distinguish between different memory types. What is the
definition of "slow"? Would all "slow" memory on the system support SMBA?

Reinette