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

[Peter Zijlstra]

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 :/