linux-kernel - Re: [PATCH v2] x86/resctrl: Fix zero cbm for AMD in cbm

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Date:   Tue, 17 May 2022 09:49:22 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Stephane Eranian <eranian@...gle.com>
CC:     <linux-kernel@...r.kernel.org>, <babu.moger@....com>,
        <x86@...nel.org>
Subject: Re: [PATCH v2] x86/resctrl: Fix zero cbm for AMD in cbm_validate

Hi Fenghua,

On 5/17/2022 9:33 AM, Fenghua Yu wrote:
> Hi, Eranian,
> 
> On Mon, May 16, 2022 at 05:12:34PM -0700, Stephane Eranian wrote:
>> AMD supports cbm with no bits set as reflected in rdt_init_res_defs_amd() by:
> ...
>> @@ -107,6 +107,10 @@ static bool cbm_validate(char *buf, u32 *data, struct rdt_resource *r)
>>  	first_bit = find_first_bit(&val, cbm_len);
>>  	zero_bit = find_next_zero_bit(&val, cbm_len, first_bit);
>>  
>> +	/* no need to check bits if arch supports no bits set */
>> +	if (r->cache.arch_has_empty_bitmaps && val == 0)
>> +		goto done;
>> +
>>  	/* Are non-contiguous bitmaps allowed? */
>>  	if (!r->cache.arch_has_sparse_bitmaps &&
>>  	    (find_next_bit(&val, cbm_len, zero_bit) < cbm_len)) {
>> @@ -119,7 +123,7 @@ static bool cbm_validate(char *buf, u32 *data, struct rdt_resource *r)
>>  				    r->cache.min_cbm_bits);
>>  		return false;
>>  	}
>> -
>> +done:
>>  	*data = val;
>>  	return true;
>>  }
> 
> Isn't it AMD supports 0 minimal CBM bits? Then should set its min_cbm_bits as 0.
> Is the following patch a better fix? I don't have AMD machine and cannot
> test the patch.
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 6055d05af4cc..031d77dd982d 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -909,6 +909,7 @@ static __init void rdt_init_res_defs_amd(void)
>  			r->cache.arch_has_sparse_bitmaps = true;
>  			r->cache.arch_has_empty_bitmaps = true;
>  			r->cache.arch_has_per_cpu_cfg = true;
> +			r->cache.min_cbm_bits = 0;
>  		} else if (r->rid == RDT_RESOURCE_MBA) {
>  			hw_res->msr_base = MSR_IA32_MBA_BW_BASE;
>  			hw_res->msr_update = mba_wrmsr_amd;

That is actually what Stephane's V1 [1] did and I proposed that
he fixes it with (almost) what he has in V2 (I think the check
can be moved earlier before any bits are searched for).

The reasons why I proposed this change are:
- min_cbm_bits is a value that is exposed to user space and from the
  time AMD was supported this has always been 1 for those systems. I
  do not know how user space uses this value and unless I can be certain
  making this 0 will not affect user space I would prefer not to
  make such a change.

- this fix restores original behavior that was changed in the patch noted
  in the Fixes link.

- this fix itself relies on math on error returns of bit checking on an empty
  bitmap. I find that hides what the code does and this fix is more obvious.
  You can see this feedback in my response to V1. 

- a fix like the above snippet is incomplete. To be appropriate 
  the initialization of rdt_resources_all[] needs to be changed to
  not initialize min_cbm_bits anymore and move the platform specific bits
  to rdt_init_res_defs_amd() and rdt_init_res_defs_intel() respectively.


Reinette

[1] https://lore.kernel.org/lkml/20220516055055.2734840-1-eranian@google.com/