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

[Sandipan Das]

On 7/19/2023 12:58 PM, Peter Zijlstra wrote:
> On Wed, Jul 19, 2023 at 12:25:38PM +0530, 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.
>>
> 
> So I hate all this for multiple reasons:
> 
>  - the wohle foo_on_cpu() model generally leads to atrocious code that
>    does multiple IPIs, I've seen rdmsr_on_cpu() followed by
>    wrmsr_on_cpu() and worse things, just don't do this.
> 
>  - The whole CPUID thing is insane; we should read CPUID -- all of it --
>    *ONCE* at bringup and thereafter never touch the instruction ever
>    again. It could be people are already working on patches to this
>    effect.
> 
>  - Different CPUID values for different CPUs is a pain :/

Thanks for the clarification. Will remove this.

- Sandipan