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

[Sandipan Das]

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@xxxxxxxxxxxxx
>>
>> 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