linux-kernel - Re: [PATCH v2 1/4] x86/resctrl: Enable non-contiguous bits in Intel CAT

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Date:   Thu, 28 Sep 2023 08:53:54 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     <babu.moger@....com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] x86/resctrl: Enable non-contiguous bits in Intel
 CAT

Hi Babu,

On 9/28/2023 8:08 AM, Moger, Babu wrote:
> On 9/28/23 02:06, Maciej Wieczór-Retman wrote:
>> On 2023-09-27 at 17:34:27 -0500, Moger, Babu wrote:
>>> On 9/22/2023 3:48 AM, Maciej Wieczor-Retman wrote:
...

>>>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
>>>> index 030d3b409768..c783a873147c 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>>>> @@ -152,6 +152,7 @@ static inline void cache_alloc_hsw_probe(void)
>>>>   	r->cache.cbm_len = 20;
>>>>   	r->cache.shareable_bits = 0xc0000;
>>>>   	r->cache.min_cbm_bits = 2;
>>>> +	r->cache.arch_has_sparse_bitmaps = false;
>>>
>>> Is this change required?
>>>
>>> This is always set to false in rdt_init_res_defs_intel().
>>
>> The logic behind moving this variable initialization from
>> rdt_init_res_defs_intel() into both cache_alloc_hsw_probe() and
>> rdt_get_cache_alloc_cfg() is that the variable doesn't really have a
>> default value anymore. It used to when the CPUID.0x10.1:ECX[3] and
>> CPUID.0x10.2:ECX[3] bits were reserved.
>>
>> Now for the general case the variable is dependent on CPUID output.
>> And only for Haswell case it needs to be hardcoded to "false", so the
>> assignment makes more sense in Haswell probe rather than in the default
>> section.
> 
> Here is the current sequence order with your change.
> 
> 1.
> resctrl_late_init -> check_quirks -> __check_quirks_intel ->
> cache_alloc_hsw_probe
>    r->cache.arch_has_sparse_bitmaps = false; (new code)
> 
> 2. resctrl_late_init -> rdt_init_res_defs -> rdt_init_res_defs_intel
>    r->cache.arch_has_sparse_bitmaps = false; (old code)
> 
> 3. resctrl_late_init -> get_rdt_resources -> get_rdt_alloc_resources ->
> rdt_get_cache_alloc_cfg
>    r->cache.arch_has_sparse_bitmaps = ecx.split.noncont; (new code)
> 
> The code in (3) is going to overwrite whatever is set in (1) or (2).
> 
> I would say you can just remove initialization in both (1) and (2). That
> makes the code clearer to me. I assume reserved bits in Intel is always 0.
> 

I believe Maciej already addressed this in his response to a similar question
from Peter. Please see:
https://lore.kernel.org/lkml/xnjmmsj5pjskbqeynor2ztha5dmkhxa44j764ohtjhtywy7idb@soobjiql4liy/

Reinette