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

["Moger, Babu" <bmoger@amd.com>]

Hi Reinette,

On 4/8/2025 8:41 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 4/8/25 5:41 PM, Moger, Babu wrote:
>> Hi Reinette,
>>
>> On 4/8/2025 4:44 PM, Reinette Chatre wrote:
>>> 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?
>>
>> I agree the text is pretty generic. Actually, the whole bit mask (0xfffF) is shareable with io_alloc.
> 
> I think the value of "shareable_bits" presented to user space could be the
> actual io_alloc_cbm value. Thus, not the "possible IO bitmask" but the actual

Confused little bit here. The shareable_bits is resource property.
io_alloc_cbm is domain specific value. Not sure how to display it.

> value. This seems to be the best match of what "shareable_bits" represents, which
> is the region currently used by IO devices. This partners well with the "bit_usage"
> output, for example, "X" can be used to show which portions of cache are currently
> used by both software (via schemata of resource groups) and hardware (via io_alloc_cbm).

Haven't looked at this code much. Will look into it.
  >>
>> The 'shareable_bits' is coming from CPUID Fn0000_0010_EBX_x1[L3ShareAllocMask] which is always 0 on AMD systems.
>> It will be bit odd to manipulate these value. Not sure if we have to do it.
> 
> It is not clear to me what you mean with "manipulate". "shareable_bits" does currently
> come from the existing register but AMD now provides more interfaces with which this data
> can be obtained and it seems appropriate to use it.

Ok.

Thanks
Babu