linux-kernel - Re: [PATCH v8 08/25] x86/resctrl: Introduce interface to display number of monitoring counters

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Message-ID: <ee0352c2-1869-44ab-94e0-a6e81fce9b4e@amd.com>
Date: Mon, 14 Oct 2024 15:32:02 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 "Luck, Tony" <tony.luck@...el.com>
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Subject: Re: [PATCH v8 08/25] x86/resctrl: Introduce interface to display
 number of monitoring counters

Hi Reinette/Tony,

On 10/14/24 15:05,  wrote:
> Hi Tony,
> 
> On 10/14/24 12:51 PM, Luck, Tony wrote:
>>>> What advantage does it have over skipping the per-domain list and
>>>> just providing a single value for all domains? You clearly expect this
>>>> will be a common user request since you implemented the "*" means
>>>> apply to all domains.
>>>>
>>>
>>> We started with a global assignment by applying assignment across all the
>>> domains initially.
>>>
>>> But we wanted give a generic approach which allows both the options(domain
>>> specific assignment and global assignment with '*"). It is also matches
>>> with other managements (RMID/CLOSID management) we are doing in resctrl
>>> right now. Also, there is an extra IPI for each domain if user is only
>>> interested in on domain.
>>>
>>> Some of the discussions are here.
>>> https://lore.kernel.org/lkml/f7dac996d87b4144e4c786178a7fd3d218eaebe8.1711674410.git.babu.moger@amd.com/#r
>>
>> My summary of that:
>>
>> Peter: Complex, don't need per-domain.
>> Reinette: Maybe some architecture might want per-domain.
> 
> To be specific ... we already have an architecture that supports per-domain:
> AMD's ABMC. When I considered the lifetime of user interfaces (forever?) while knowing
> that ABMC does indeed support per-domain counter assignment it seems a good
> precaution for the user interface to support that, even if the first
> implementation does not.
> 
> There are two parts to this work: (a) the new user interface
> and (b) support for ABMC. I believe that the user interface has to be
> flexible to support all ABMC features that users may want to take advantage of,
> even if the first implementation does not enable those features. In addition,
> the user interface should support future usages that we know if, "soft-ABMC"
> and MPAM.
> 
> I do not think that we should require all implementations to support everything
> made possible by user interface though. As I mentioned in that thread [1] I do
> think that the user _interface_ needs to be flexible by supporting domain level
> counter assignment, but that it may be possible that the _implementation_ only
> supports assignment to '*' domain values. 
> 
> I thus do not think we should simplify the syntax of mbm_assign_control,
> but I also do not think we should require that all implementations support all that
> the syntax makes possible. 
>  
>> Since you seem to want to keep the flexibility for a possible future
>> where per-domain is needed. The "available_mbm_cntrs" file
>> suggested in another thread would need to list available counters
>> on each domain to avoid ABI problems should that future arrive.
>>
>> $ cat num_mbm_counters
>> 32
>>
>> $ cat available_mbm_cntrs
>> 0=12;1=9
> 
> Good point.

Ok. Will add it.
Thanks
Babu Moger