Re: [PATCH v3 0/7] Support L3 Smart Data Cache Injection Allocation Enforcement (SDCIAE)

[Reinette Chatre]

Hi Babu,

On 4/7/25 1:12 PM, Moger, Babu wrote:
> On 3/21/25 17:50, Reinette Chatre wrote:
>> On 1/30/25 1:20 PM, Babu Moger wrote:


>>>
>>
>> AMD also supports what is exposed to user space as "shareable_bits". According
>> to APM:
>> 	Depending on the implementation, some portions of the L3 Cache may be
>> 	shared by other system functions or used for some other purpose not
>> 	under the control of the PQOS feature set. The L3 Cache Allocation
>> 	Sharing Mask returned by CPUID Fn0000_0010_EBX_x1[L3ShareAllocMask] is a
>> 	bitmask that represents portions of the L3 that may be shared by those
>> 	functions.
> 
> Here is the complete text.
> 
> The L3 Cache allocation sharing mask (L3ShareAllocMask) returned in EBX by
> CPUID Fn0000_0010 with ECX=1 is a bit vector which represents portions of
> the cache which may be shared with other system entities or used for some
> other purpose not under the control of the QOS feature set. When software
> sets a bit in one of the L3_MASK_n registers at the same bit positions a
> bit in the L3ShareAllocMask, processors executing with the corresponding
> COS will competitively share that portion of the cache with the other
> function. If this mask is all 0’s, then there is no other entity in the
> system competing with the processors for use of the L3 cache.
> 
> The "L3ShareAllocMask" is always reported as 0 on AMD systems.
> 
>> Could you please include what (if any) the relationship is between the CBM
>> discoverable via Fn0000_0010_EBX_x1[L3ShareAllocMask] and the CBM of 
>> "highest-supported L3_MASK_n register" when SDCIAE is enabled?
> 
> No. There is no relationship in here.
> 
>>
>> On the resctrl interface side the documentation currently states:
>>
>> 	"shareable_bits":                                                               
>> 		Bitmask of shareable resource with other executing              
>> 		entities (e.g. I/O). User can use this when                     
>> 		setting up exclusive cache partitions. Note that                
>> 		some platforms support devices that have their                  
>> 		own settings for cache use which can over-ride                  
>> 		these bits.                             
>>
>> Even though this was originally used to expose the content of
>> Fn0000_0010_EBX_x1[L3ShareAllocMask] the intent of the content does
>> seem to also apply to the "io_alloc" CBM also.
> 
> It says "shared by other system functions or used for some other purpose
> not under the control of the PQOS feature set".

This is a quote from the AMD spec, not the resctrl user interface documentation.

Please consider this from resctrl user interface perspective.

> 
> "io_alloc" is PQOS feature set. I feel it should not affect "shareable_bits".

When I read the resctrl user interface documentation for "shareable_bits" it
sounds relevant to io_alloc. The "shareable_bits" contains a bitmask "of
shareable resource with other executing entities (e.g. I/O)" ... is this
not exactly io_alloc? 


Reinette