linux-kernel - Re: [PATCH] ACPI/HMAT: Fix the parsing of Cache Associativity and Write Policy

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Date:   Tue, 10 Dec 2019 21:18:47 +0800
From:   Tao Xu <tao3.xu@...el.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI/HMAT: Fix the parsing of Cache Associativity and
 Write Policy

On 12/10/2019 4:27 PM, Rafael J. Wysocki wrote:
> On Tue, Dec 10, 2019 at 9:19 AM Tao Xu <tao3.xu@...el.com> wrote:
>>
>> On 12/10/2019 4:06 PM, Rafael J. Wysocki wrote:
>>> On Tue, Dec 10, 2019 at 2:04 AM Tao Xu <tao3.xu@...el.com> wrote:
>>>>
>>>> On 12/9/2019 6:01 PM, Rafael J. Wysocki wrote:
>>>>> On Mon, Dec 2, 2019 at 8:03 AM Tao Xu <tao3.xu@...el.com> wrote:
>>>>>>
>>>>>> In chapter 5.2.27.5, Table 5-147: Field "Cache Attributes" of
>>>>>> ACPI 6.3 spec: 0 is "None", 1 is "Direct Mapped", 2 is "Complex Cache
>>>>>> Indexing" for Cache Associativity; 0 is "None", 1 is "Write Back",
>>>>>> 2 is "Write Through" for Write Policy.
>>>>>
>>>>> Well, I'm not sure what the connection between the above statement,
>>>>> which is correct AFAICS, and the changes made by the patch is.
>>>>>
>>>>> Is that the *_OTHER symbol names are confusing or something deeper?
>>>>>
>>>>
>>>> Because in include/acpi/actbl1.h:
>>>>
>>>> #define ACPI_HMAT_CA_NONE                     (0)
>>>>
>>>> ACPI_HMAT_CA_NONE is 0, but in include/linux/node.h:
>>>>
>>>>       enum cache_indexing {
>>>>              NODE_CACHE_DIRECT_MAP,
>>>>              NODE_CACHE_INDEXED,
>>>>              NODE_CACHE_OTHER,
>>>>       };
>>>> NODE_CACHE_OTHER is 2, and for otner enum:
>>>>
>>>>             case ACPI_HMAT_CA_DIRECT_MAPPED:
>>>>                     tcache->cache_attrs.indexing = NODE_CACHE_DIRECT_MAP;
>>>>                     break;
>>>>             case ACPI_HMAT_CA_COMPLEX_CACHE_INDEXING:
>>>>                     tcache->cache_attrs.indexing = NODE_CACHE_INDEXED;
>>>>                     break;
>>>> in include/acpi/actbl1.h:
>>>>
>>>>     #define ACPI_HMAT_CA_DIRECT_MAPPED            (1)
>>>>     #define ACPI_HMAT_CA_COMPLEX_CACHE_INDEXING   (2)
>>>>
>>>> but in include/linux/node.h:
>>>>
>>>> NODE_CACHE_DIRECT_MAP is 0, NODE_CACHE_INDEXED is 1. This is incorrect.
>>>
>>> Why is it incorrect?
>>
>> Sorry I paste the wrong pre-define.
>>
>> This is the incorrect line:
>>
>> case ACPI_HMAT_CA_DIRECT_MAPPED:
>> tcache->cache_attrs.indexing = NODE_CACHE_DIRECT_MAP;
>>
>> ACPI_HMAT_CA_DIRECT_MAPPED is 1, NODE_CACHE_DIRECT_MAP is 0. That means
>> if HMAT table input 1 for cache_attrs.indexing, kernel store 0 in
>> cache_attrs.indexing. But in ACPI 6.3, 0 means "None". So for the whole
>> switch codes:
> 
> This is a mapping between the ACPI-defined values and the generic ones
> defined in the kernel.  There is not rule I know of by which they must
> be the same numbers.  Or is there such a rule which I'm missing?
> 
> As long as cache_attrs.indexing is used consistently going forward,
> the difference between the ACPI-defined numbers and its values
> shouldn't matter, should it?
> 
Yes, it will not influence the ACPI HMAT tables. Only influence is the 
sysfs, as in 
https://www.kernel.org/doc/html/latest/admin-guide/mm/numaperf.html:

# tree sys/devices/system/node/node0/memory_side_cache/
/sys/devices/system/node/node0/memory_side_cache/
|-- index1
|   |-- indexing
|   |-- line_size
|   |-- size
|   `-- write_policy

indexing is parsed in this file, so it can be read by user-space. 
Although now there is no user-space tool use this information to do some 
thing. But I am wondering if it is used in the future, someone use it to 
show the memory side cache information to user or use it to do 
performance turning.