linux-kernel - Re: [PATCH v6 01/22] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <87le182t1a.ffs@tglx>
Date: Wed, 07 Aug 2024 18:32:01 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Babu Moger <babu.moger@....com>, corbet@....net, fenghua.yu@...el.com,
 reinette.chatre@...el.com, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, paulmck@...nel.org,
 rdunlap@...radead.org, tj@...nel.org, peterz@...radead.org,
 yanjiewtw@...il.com, babu.moger@....com, kim.phillips@....com,
 lukas.bulwahn@...il.com, seanjc@...gle.com, jmattson@...gle.com,
 leitao@...ian.org, jpoimboe@...nel.org, rick.p.edgecombe@...el.com,
 kirill.shutemov@...ux.intel.com, jithu.joseph@...el.com,
 kai.huang@...el.com, kan.liang@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, pbonzini@...hat.com, sandipan.das@....com,
 ilpo.jarvinen@...ux.intel.com, peternewman@...gle.com,
 maciej.wieczor-retman@...el.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, eranian@...gle.com, james.morse@....com
Subject: Re: [PATCH v6 01/22] x86/cpufeatures: Add support for Assignable
 Bandwidth Monitoring Counters (ABMC)

On Tue, Aug 06 2024 at 17:00, Babu Moger wrote:
> Users can create as many monitor groups as RMIDs supported by the hardware.
> However, bandwidth monitoring feature on AMD system only guarantees that
> RMIDs currently assigned to a processor will be tracked by hardware. The
> counters of any other RMIDs which are no longer being tracked will be
> reset to zero. The MBM event counters return "Unavailable" for the RMIDs
> that are not tracked by hardware. So, there can be only limited number of
> groups that can give guaranteed monitoring numbers. With ever changing
> configurations there is no way to definitely know which of these groups
> are being tracked for certain point of time. Users do not have the option
> to monitor a group or set of groups for certain period of time without
> worrying about RMID being reset in between.
>
> The ABMC feature provides an option to the user to assign a hardware
> counter to an RMID and monitor the bandwidth as long as it is assigned.
> The assigned RMID will be tracked by the hardware until the user unassigns
> it manually. There is no need to worry about counters being reset during
> this period. Additionally, the user can specify a bitmask identifying the
> specific bandwidth types from the given source to track with the counter.
>
> Without ABMC enabled, monitoring will work in current mode without
> assignment option.
>
> Linux resctrl subsystem provides the interface to count maximum of two
> memory bandwidth events per group, from a combination of available total
> and local events. Keeping the current interface, users can enable a maximum
> of 2 ABMC counters per group. User will also have the option to enable only
> one counter to the group. If the system runs out of assignable ABMC
> counters, kernel will display an error. Users need to disable an already
> enabled counter to make space for new assignments.
>
> The feature can be detected via CPUID_Fn80000020_EBX_x00 bit 5.
> Bits Description
> 5    ABMC (Assignable Bandwidth Monitoring Counters)

Can you please update the CPUID database with that new bit:

    https://gitlab.com/x86-cpuid.org/x86-cpuid-db

Thanks,

        tglx