linux-kernel - Re: [PATCH v1 17/31] x86/resctrl: Move mbm_cfg_mask to struct rdt

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Date: Wed, 17 Apr 2024 22:18:48 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, Fenghua Yu <fenghua.yu@...el.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Borislav
 Petkov" <bp@...en8.de>, H Peter Anvin <hpa@...or.com>, Babu Moger
	<Babu.Moger@....com>, <shameerali.kolothum.thodi@...wei.com>, "D Scott
 Phillips OS" <scott@...amperecomputing.com>, <carl@...amperecomputing.com>,
	<lcherian@...vell.com>, <bobo.shaobowang@...wei.com>,
	<tan.shaopeng@...itsu.com>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>
Subject: Re: [PATCH v1 17/31] x86/resctrl: Move mbm_cfg_mask to struct
 rdt_resource

Hi Dave,

On 4/17/2024 7:41 AM, Dave Martin wrote:
> On Thu, Apr 11, 2024 at 10:39:06AM -0700, Reinette Chatre wrote:
>> Hi Dave,
>>
>> On 4/11/2024 7:16 AM, Dave Martin wrote:
>>> On Mon, Apr 08, 2024 at 08:21:24PM -0700, Reinette Chatre wrote:
>>>> Hi James,
>>>>
>>>> On 3/21/2024 9:50 AM, James Morse wrote:
>>>>> The mbm_cfg_mask field lists the bits that user-space can set when
>>>>> configuring an event. This value is output via the last_cmd_status
>>>>> file.
>>>>>
>>>>> Once the filesystem parts of resctrl are moved to live in /fs/, the
>>>>> struct rdt_hw_resource is inaccessible to the filesystem code. Because
>>>>> this value is output to user-space, it has to be accessible to the
>>>>> filesystem code.
>>>>>
>>>>> Move it to struct rdt_resource.
>>>>>
>>>>> Signed-off-by: James Morse <james.morse@....com>
>>>>
>>>> ..
>>>>
>>>>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>>>>> index 975b80102fbe..8a7367d1ce45 100644
>>>>> --- a/include/linux/resctrl.h
>>>>> +++ b/include/linux/resctrl.h
>>>>> @@ -140,6 +140,8 @@ struct resctrl_membw {
>>>>>   * @format_str:		Per resource format string to show domain value
>>>>>   * @evt_list:		List of monitoring events
>>>>>   * @fflags:		flags to choose base and info files
>>>>> + * @mbm_cfg_mask:	Bandwidth sources that can be tracked when Bandwidth
>>>>> + *			Monitoring Event Configuration (BMEC) is supported.
>>>
>>> [...]
>>>
>>>> Reinette
>>>>
>>>> BMEC is an AMD term. If MPAM is planning to use this member then this comment
>>>> should be made generic.
>>>
>>> MPAM can (at least) filter reads and/or writes, and it looks like James
>>> is expecting to support this.
>>>
>>> However, it probably does make sense to keep the comments neutral in a
>>> common header.
>>>
>>> Would the following work?
>>>
>>> 	* @ mbm_cfg_mask:	Arch-specific event configuration flags
>>>
>>>
>>>
>>> I think that's broad enough to cover all bases, but I'll wait for your
>>> response.
>>
>> Since this is exposed to user space, ideally all architectures would use
>> the same bits to mean the same thing. I assumed that is what James intended
>> by placing the existing (AMD BMEC) bits in the global resctrl_types.h.
>>
>> Reinette
> 
> Maybe, but the bits as defined by AMD BMEC look rather architecture and
> bus specific, and I am suspicious that there is no guaranteed clean
> mapping between MPAM's config and BMEC's config.
> 
> MPAM currently just has "reads" and "writes" (or both), though as for
> BMEC, the meanings of these are not fully defined.  I suppose finer
> filtering granularity might be supported in future (at least, it is not
> explicitly ruled out).
> 
> James' current approach seems to be to pick a single BMEC flag that's
> in the right sort of area for each MPAM bit (though not equivalent) and
> translate that bit across to drive a corresponding the MPAM bit.  But
> I'd say that this is arch-specific configuration masquerading as
> generic configuration IMHO and not really generic at all.
> 
> See "untested: arm_mpam: resctrl: Allow monitors to be configured"
> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/commit/?h=mpam/snapshot/v6.7-rc2&id=db0ac51f60675b6c4a54ccd24fa7198ec321c56d
> 
> I guess this needs discussion with James, since there will have been
> additional thought process behind all this that is not captured; either
> way, I guess it could be resolved after this series, but it will need a
> decision before the MPAM support is merged (or at least, before MPAM
> exposes userspace support for event configuration upstream).
> 
> (If this has already been discussed and James' current approach has
> already been agreed as the least worst option, then I guess I can live
> with it; I just find it icky, and it looks odd to have AMD specifics in
> a common header.)

I am not aware of such a discussion.

Sounds like a motivation to delay this portion of the changes in patch #8.

Reinette