[RFC] arm-mpam: how to define mpam cache reference

[Wangshaobo (bobo)]

Thanks a lot.

- Wang ShaoBo

在 2023/6/6 0:27, James Morse 写道:
> Hi Wang ShaoBo,
> 
> (CC: +linux-arm-kernel at lists.infradead.org ... linux-arm at vger.kernel.org isn't a thing)
> 
> On 05/06/2023 11:23, Wangshaobo (bobo) wrote:
>> I checked the protocol specification of MPAM ACPI V2 from https://developer.arm.com
>> /documentation/den0065/latest锟斤拷
>> The explanation for the cache locator part is as following:
>>
>> ```
>> The cache locator is described in terms of the cache锟斤拷s topological position within
>> the processor hierarchy.
>>
>> The position is derived from the ACPI PPTT table [2] , and is expressed as a reference to
>> the cache (Type 1) structure within the PPTT table that describes the cache...
>>
>> ...
>> Cache reference : This field must match the Identifier field of the
>> PPTT Type 1 structure that describes this cache.
>>
>> ```
>>
>> And your code branch (https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git)
>> directly uses the cache id to index the location of the cache node, and also requires PPTT
>> revision >=3:
> 
> [...]
> 
>> So if my PPTT revision < 3, could we traverse all cache nodes in PPTT by giving an offset?
> 
> Absolutely not. Having a byte offset between tables is an nightmare to work with. Your
> UEFI firmware may generate or modify the tables at boot. Having byte-offsets between them
> means you end up with a tangled web of tables that have to be re-generated. The OS then
> has to parse the full table every time to check the byte-offset lands on a sensible boundary.
> 
> Please update your PPTT to support identifying the caches. MPAM depends on this to which
> cache is which.
> 
> 
>> and to get the cache level and finally link all cache nodes with the same PPTT_ref to same
>> mpam component,this is your earlier code:
> 
> Yup, I wrote this while arguing with the spec folk that we shouldn't do this. It never
> made it into a published version of the ACPI spec.
> 
> 
> 
> Thanks,
> 
> James
> 
> 
> .
>