linux-kernel - Re: [PATCH 3/6] x86/cpuid: Add smp helper

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Message-ID: <b42e64dd-a404-7118-798b-f63c0a725962@amd.com>
Date:   Wed, 19 Jul 2023 13:05:29 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Thomas Gleixner <tglx@...utronix.de>, peterz@...radead.org
Cc:     x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, irogers@...gle.com,
        adrian.hunter@...el.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, eranian@...gle.com, ananth.narayan@....com,
        ravi.bangoria@....com, santosh.shukla@....com
Subject: Re: [PATCH 3/6] x86/cpuid: Add smp helper

On 7/19/2023 12:59 PM, Thomas Gleixner wrote:
> On Wed, Jul 19 2023 at 12:25, Sandipan Das wrote:
>> Depending on which CPU the CPUID instruction is executed, some leaves
>> can report different values. There are cases where it may be required
>> to know all possible values.
>>
>> E.g. for AMD Zen 4 processors, the ActiveUmcMask field from leaf
>> 0x80000022 ECX, which provides a way to determine the active memory
>> controllers, can have different masks on CPUs belonging to different
>> sockets as each socket can follow a different DIMM population scheme.
>> Each memory channel is assigned a memory controller (UMC) and if no
>> DIMMs are attached to a channel, the corresponding memory controller
>> is inactive. There are performance monitoring counters exclusive to
>> each memory controller which need to be represented under separate
>> PMUs. So, it will be necessary to know the active memory controllers
>> on each socket during the initialization of the UMC PMUs irrespective
>> of where the uncore driver's module init runs.
>>
>> Add a new helper that executes CPUID on a particular CPU and returns
>> the EAX, EBX, ECX and EDX values.
> 
> NAK.
> 
> This madness has to stop. The correct thing is to parse the information
> in CPUID at the point where the CPU comes online and store it for easy
> consumption.
> 
> I'm in the process of reworking the CPUID and topology evaluation and
> that's where these things need to be stored. I'm still fighting some
> nightmares with the already existing mess.
> 
> Look at the mess people created over time here:
> 
>      https://lore.kernel.org/lkml/20230717223049.327865981@linutronix.de
> 
> No need to add more insanities to it. IOW, this has to wait for a week
> or two until I settled the remaining issues.
> 

Agreed. I'll rework the patches and remove this.

- Sandipan