linux-kernel - Re: [PATCH v13 01/27] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <3c22ea8f-7467-4a25-b1b7-4f7f47177211@amd.com>
Date: Tue, 27 May 2025 13:40:30 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: james.morse@....com, dave.martin@....com, fenghuay@...dia.com,
 x86@...nel.org, hpa@...or.com, paulmck@...nel.org,
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 ardb@...nel.org, gregkh@...uxfoundation.org, daniel.sneddon@...ux.intel.com,
 jpoimboe@...nel.org, alexandre.chartre@...cle.com,
 pawan.kumar.gupta@...ux.intel.com, thomas.lendacky@....com,
 perry.yuan@....com, seanjc@...gle.com, kai.huang@...el.com,
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Subject: Re: [PATCH v13 01/27] x86/cpufeatures: Add support for Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Reinette,

On 5/27/25 12:54, Reinette Chatre wrote:
> Hi Babu,
> 
> On 5/27/25 10:23 AM, Moger, Babu wrote:
>> On 5/22/25 15:51, Reinette Chatre wrote:
>>> On 5/15/25 3:51 PM, Babu Moger wrote:
> 
>>>> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
>>>> index a2fbea0be535..2f54831e04e5 100644
>>>> --- a/arch/x86/kernel/cpu/cpuid-deps.c
>>>> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
>>>> @@ -71,6 +71,8 @@ static const struct cpuid_dep cpuid_deps[] = {
>>>>  	{ X86_FEATURE_CQM_MBM_LOCAL,		X86_FEATURE_CQM_LLC   },
>>>>  	{ X86_FEATURE_BMEC,			X86_FEATURE_CQM_MBM_TOTAL   },
>>>>  	{ X86_FEATURE_BMEC,			X86_FEATURE_CQM_MBM_LOCAL   },
>>>> +	{ X86_FEATURE_ABMC,			X86_FEATURE_CQM_MBM_TOTAL   },
>>>> +	{ X86_FEATURE_ABMC,			X86_FEATURE_CQM_MBM_LOCAL   },
>>>
>>> Is this dependency still accurate now that the implementation switched to the 
>>> "extended event ID" variant of ABMC that no longer uses the event IDs associated
>>> with X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL?
>>
>> That's a good question. Unfortunately, we may need to retain this
>> dependency for now, as a significant portion of the code relies on
>> functions like resctrl_is_mbm_event(), resctrl_is_mbm_enabled(),
>> resctrl_arch_is_mbm_total_enabled(), and others.
>>
> 
> Avoiding needing to change code is not a valid reason. 
> 
> I think that without this dependency the code will
> still rely on "functions like resctrl_is_mbm_event(),
> resctrl_is_mbm_enabled(), resctrl_arch_is_mbm_total_enabled(),
> and others." though.
> 
> The core shift is to stop thinking about QOS_L3_MBM_TOTAL_EVENT_ID
> to mean the same as X86_FEATURE_CQM_MBM_TOTAL, similarly to stop
> thinking about QOS_L3_MBM_LOCAL_EVENT_ID to mean the same as
> X86_FEATURE_CQM_MBM_LOCAL.

oh. ok.

> 
> I expected that for backwards compatibility ABMC will start by
> enabling QOS_L3_MBM_TOTAL_EVENT_ID and QOS_L3_MBM_LOCAL_EVENT_ID 
> as part of its initialization, configuring them with the current
> defaults for which memory transactions are expected to be monitored
> by each. With these events enabled the existing flows using, for
> example, resctrl_is_mbm_event(), will continue to work as expected, no?

Yes. It will work as it uses event id.
> 
> This would require more familiarity with L3 monitoring enumeration
> on AMD since it will still be required to determine the number of
> RMIDs etc. but if ABMC does not actually depend on these CQM features
> then the current enumeration would need to be re-worked anyway.

Are you suggesting to remove the dependency and rework ABMC enumeration in
get_rdt_mon_resources()?

-- 
Thanks
Babu Moger