linux-kernel - Re: [PATCH v11 04/13] x86/cpufeatures: Add Bandwidth Monitoring Event Configuration feature flag

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Date:   Mon, 9 Jan 2023 15:43:11 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Borislav Petkov <bp@...en8.de>, "Luck, Tony" <tony.luck@...el.com>
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Subject: Re: [PATCH v11 04/13] x86/cpufeatures: Add Bandwidth Monitoring Event
 Configuration feature flag

Hi Boris and Tony,

On 1/9/2023 1:39 PM, Borislav Petkov wrote:
> On Mon, Jan 09, 2023 at 09:25:32PM +0000, Luck, Tony wrote:
>> It feels like the old "rule" was "make it visible in /proc/cpuid" unless there was some
>> good reason NOT to do it.  But that has resulted in the "flags" line getting ridiculously
>> long and hard for humans to read (141 fields with 926 bytes on my Skylake,
>> more on more modern CPUs).
> 
> Yap, imagine every possible CPUID bit were in there...
> 
>> I don't know if we'd break anything if we dropped:
>>
>>   cat_l3 cdp_l3 mba cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local
>>
>> from /proc/cpuinfo.
> 
> I wouldn't mind if we remove them from cpuinfo, frankly.

I am afraid that I am not aware of all the resctrl user space apps and
tools being used.

I did take a quick look at intel-cmt-cat that has an active user base and
from what I can tell it uses /proc/cpuinfo to learn some capabilities:

Example of looking for "cqm" (although "cqm" is not in Tony's list):
https://github.com/intel/intel-cmt-cat/blob/master/lib/os_cap.c#L420

Example of looking for "cdp_l3":
https://github.com/intel/intel-cmt-cat/blob/master/lib/os_cap.c#L520

Example of looking for "cdp_l2":
https://github.com/intel/intel-cmt-cat/blob/master/lib/os_cap.c#L564

> 
>> Perhaps the "rule" should be written in Documentation/{somewhere}?
> 
> This started documenting it:
> 
> Documentation/x86/cpuinfo.rst
> 

We could make a rule that no more resctrl related features are added to
cpuinfo but I am hesitant to remove the ones that are already there. 

Reinette