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

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Message-ID: <46e40771-c73c-4ceb-acc3-812693f98f19@amd.com>
Date: Tue, 8 Apr 2025 19:41:22 -0500
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, babu.moger@....com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, akpm@...ux-foundation.org,
 paulmck@...nel.org, thuth@...hat.com, rostedt@...dmis.org,
 xiongwei.song@...driver.com, pawan.kumar.gupta@...ux.intel.com,
 jpoimboe@...nel.org, daniel.sneddon@...ux.intel.com,
 thomas.lendacky@....com, perry.yuan@....com, sandipan.das@....com,
 kai.huang@...el.com, seanjc@...gle.com, xin3.li@...el.com,
 ebiggers@...gle.com, andrew.cooper3@...rix.com, mario.limonciello@....com,
 tan.shaopeng@...itsu.com, james.morse@....com, tony.luck@...el.com,
 peternewman@...gle.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, eranian@...gle.com, corbet@....net
Subject: Re: [PATCH v3 0/7] Support L3 Smart Data Cache Injection Allocation
 Enforcement (SDCIAE)

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.

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.

Thanks
Babu