Re: [PATCH v2] x86/resctrl: Fix zero cbm for AMD in cbm_validate

[Reinette Chatre]

Hi Fenghua,

On 5/17/2022 10:27 AM, Fenghua Yu wrote:
> Hi, Reinette,
> 
> On Tue, May 17, 2022 at 09:49:22AM -0700, Reinette Chatre wrote:
>> 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.
> 
> But a user visible mismatch is created by the V2 patch:

No. V2 does not create a user visible change, it restores original behavior.

> User queries min_cbm_bits and finds it is 1 but turns out 0 can be written
> to the schemata.
> 
> Is it an acceptable behavior? Shouldn't user read right min_cbm_bits (0)
> on AMD?

Original AMD enabling set min_cbm_bits as 1 while also supporting 0 to
be written to schemata file. Supporting 0 to be written to schemata file
was unintentionally broken during one of the MPAM prep patches. This
patch fixes that.

You are proposing a change to original AMD support that impacts user
space API while I find fixing to restore original behavior to
be the safest option. Perhaps Babu could pick the preferred solution
and I would step aside if you prefer to go with (an improved) V1.

Reinette