linux-kernel - Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch

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Message-ID: <38569247-431c-4b0c-af0f-aadd6fb26849@intel.com>
Date: Wed, 1 Oct 2025 20:13:45 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: "Luck, Tony" <tony.luck@...el.com>, <linux-kernel@...r.kernel.org>,
	Reinette Chatre <reinette.chatre@...el.com>, James Morse
	<james.morse@....com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
	<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, "Jonathan
 Corbet" <corbet@....net>, <x86@...nel.org>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be
 per-arch

On 9/30/2025 11:55 PM, Dave Martin wrote:
> Hi,
> 
> On Tue, Sep 30, 2025 at 12:43:36PM +0800, Chen, Yu C wrote:
>> On 9/29/2025 10:13 PM, Dave Martin wrote:
> 
> [...]
> 
>>> I guess it would be up to the arch code whether to trust ACPI if it
>>> says that the maximum value of this field is > 511.  (> 65535 would be
>>> impossible though, since the fields would start to overlap each
>>> other...)
>>>
>>> Would anything break in the interface proposed here, if the maximum
>>> value is larger than 511?  (I'm hoping not.  For MPAM, the bandwidth
>>> controls can have up to 16 bits and the size can be probed though MMIO
>>> registers.
>>>
>>
>> I overlooked this bit width. It should not exceed 511 according to the
>> RDT spec. Previously, I was just wondering how to calculate the legacy
>> MB percentage in Tony's example. If we want to keep consistency - if
>> the user provides a value of 10, what is the denominator: Is it 255,
>> 511, or something queried from ACPI.
>>
>> MB: 0=4;1=75           <--- 10/255
>> #MB_HW: 0=10;1=191
>> #MB_MIN: 0=10;1=191
>> #MB_MAX: 0=64;1=191
>>
>> or
>>
>> MB: 0=1;1=75          <--- 10/511
>> #MB_HW: 0=10;1=191
>> #MB_MIN: 0=10;1=191
>> #MB_MAX: 0=64;1=191
>>
>> thanks,
>> Chenyu
> 
> The denomiator (the "scale" parameter in my model, though the name is
> unimportant) should be whatever quantity of resource is specified in
> the "unit" parameter.
> 
> For "percentage" type controls, I'd expect the unit to be 100% ("100pc"
> in my syntax).
> 
> So, Tony suggestion looks plausible to me [1] :
> 
>   | Yes. 255 (or whatever "Q" value is provided in the ACPI table)
>   | corresponds to no throttling, so 100% bandwidth.
> 
> So, if ACPI says Q=387, that's the denominator we advertise.
> 
> Does that sound right?
> 

Yes, it makes sense, the denominator is the "scale" in your example.

> Question: is this a global parameter, or per-CPU?
> 

It should be a global setting for all the MBA Register Blocks.

Thanks,
Chenyu

>  From the v1.2 RDT spec, it looks like it is a single, global parameter.
> I hope this is true (!)  But I'm not too familiar with these specs...
> 
> Cheers
> ---Dave
> 
> 
> [1] https://lore.kernel.org/lkml/aNq11fmlac6dH4pH@agluck-desk3/
>>