linux-kernel - Re: [PATCH v7 13/24] x86/resctrl: Add data structures and definitions for ABMC assignment

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Message-ID: <5093395d-d8b7-482c-825a-5b87df7aecfa@intel.com>
Date: Mon, 23 Sep 2024 15:30:05 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>, <james.morse@....com>
Subject: Re: [PATCH v7 13/24] x86/resctrl: Add data structures and definitions
 for ABMC assignment

Hi Babu,

On 9/23/24 1:21 PM, Moger, Babu wrote:
> Hi Reinette,
> 
> On 9/19/24 12:08, Reinette Chatre wrote:
>> Hi Babu,
>>
>> On 9/4/24 3:21 PM, Babu Moger wrote:
>>> +/*
>>> + * ABMC counters can be configured by writing to L3_QOS_ABMC_CFG.
>>> + * @bw_type		: Bandwidth configuration(supported by BMEC)
>>> + *			  tracked by the @cntr_id.
>>> + * @bw_src		: Bandwidth source (RMID or CLOSID).
>>> + * @reserved1		: Reserved.
>>> + * @is_clos		: @bw_src field is a CLOSID (not an RMID).
>>> + * @cntr_id		: Counter identifier.
>>> + * @reserved		: Reserved.
>>> + * @cntr_en		: Tracking enable bit.
>>> + * @cfg_en		: Configuration enable bit.
>>> + *
>>> + * Configuration and tracking:
>>> + * CfgEn=1,CtrEn=0 : Configure CtrID and but no tracking the events yet.
>>> + * CfgEn=1,CtrEn=1 : Configure CtrID and start tracking events.
>>
>> Thanks for moving the text ... could it now be made to match the new (outside
>> AMD arch document) destination? For example, "CfgEn" becomes "@cfg_en",
> 
> Sure. Will do.
> 
>> "CtrID" becomes "@cntr_id" etc. Also please fix language, for example
>> what does "and but no tracking the events yet" mean? So far this work
>> has focused on "counting" vs "not counting" events and it is not
> 
> I will change the text to "not counting".  Hope this will clarify here.
> 
>> clear how this "tracking" fits it ... this seems to be the hardware
>> view that means "tracking the RMID to which @cntr_id is assigned"?
>> Please help readers to understand how the implementation is supported
>> by the hardware.
> 
> I have checked with hw team on this.
> CfgEn: This corresponds counter assignment.

To be specific this corresponds to *hardware* counter assignment? This is
because software sets CfgEn to 1 whether it is assigned from kernel perspective
or not.

Actually ... when I look at the AMD spec it becomes more clear to me. If I
understand the spec correctly the CfgEn bit is used to coordinate changes
between OS and HW. Seems like OS can leisurely write to any fields of
L3_QOS_ABMC_CFG, but only when CfgEn bit is set will the actual hardware
configuration be performed.

> CtrEn: This is to start or stop counting.
>        We always set this to 1 to start counting.

Understood. Now that I read this portion of AMD spec it is more clear to me
and I understand why CfgEn is set in both counter assign and unassign.

Reinette