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

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Date:   Tue, 30 Aug 2022 15:23:17 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     <babu.moger@....com>
CC:     <bagasdotme@...il.com>, <bp@...en8.de>, <corbet@....net>,
        <dave.hansen@...ux.intel.com>, <eranian@...gle.com>,
        <fenghua.yu@...el.com>, <hpa@...or.com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <mingo@...hat.com>, <tglx@...utronix.de>, <tony.luck@...el.com>,
        <x86@...nel.org>
Subject: Re: [PATCH v3 02/10] x86/cpufeatures: Add Slow Memory Bandwidth
 Allocation feature flag

Hi Babu,

On 8/30/2022 3:10 PM, Moger, Babu wrote:
> On 8/30/2022 11:39 AM, Reinette Chatre wrote:

>> Would the "SMBA" resource be available only when CXL.mem devices are present
>> on the system? Since this is a CPU feature it is unclear to me whether
>> presence of CXL.mem devices would be known at the time "SMBA" is enumerated.
>> Could the "SMBA" resource thus exist without memory to throttle?
> 
> Yes.The presence of the SMBA feature(with CXL.mem) is independent of whether slow memory is actually present in the system.If there is no slow memory, then setting a SMBA limit will have no impact on the performance of the system.
> 

I think this is significant and should also be documented.

Reinette