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

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Date:   Thu, 25 Aug 2022 17:42:45 -0500
From:   "Moger, Babu" <bmoger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        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


On 8/23/2022 5:47 PM, Reinette Chatre wrote:
> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.amd.com%2Fen%2Fsupport%2Ftech-docs%2Famd64-technology-platform-quality-service-extensions&amp;data=05%7C01%7Cbabu.moger%40amd.com%7C4385e95126b24de58aec08da85597c88%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637968916632283680%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=dKSxIinxZGybtACbs3%2FVZr4zbeAvXYc%2FezVivq3xjx0%3D&amp;reserved=0
>> Link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D206537&amp;data=05%7C01%7Cbabu.moger%40amd.com%7C4385e95126b24de58aec08da85597c88%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637968916632283680%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=f7MJmrwkBGxq8BuWjNY6Ze9NdzJc6NOkXxNjUZk5c4U%3D&amp;reserved=0
>> 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
In this case the slow memory means memory attached to CXL device.
> 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
Yes. that is correct. They are seen as downstream from L3.
> 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

This memory completely transparent to OS with little bit higher latency 
that regular main memory.

Yes. I know slow word is bit vague. I am not an expert of CXL. But i see 
that word slow is being used to refer the CXL memory to differentiate it 
from regular memory.

> 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?

Yes. All the slow memory in the system can support SMBA.

Thanks

Babu

>
> Reinette