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

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Message-ID: <3273e3fa-52a7-ab23-b64f-217cc410915f@amd.com>
Date:   Wed, 19 Jul 2023 17:29:55 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        x86@...nel.org, peterz@...radead.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 5:20 PM, Thomas Gleixner wrote:
> On Wed, Jul 19 2023 at 09:29, 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.
>>> 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.
> 
> Actually you can do that today already. Either make it part of the CPUID
> evaluation or use the perf hotplug callback mechanism which is invoked
> early on the upcoming CPU and read that CPUID leaf.
> 

Thanks for the suggestions. I'll start with the hotplug callback mechanism.

- Sandipan