Re: [edk2-devel] [PATCH 1/2] UefiCpuPkg/MpInitLib: Use AsmCpuidEx() for CPUID_EXTENDED_TOPOLOGY leaf

["Ni, Ray" <ray.ni@intel.com>]

> The worry I have here is that it becomes a maintenance concern having to
> update the list of leaves with sub-leaves whenever a new CPUID leaf with
> sub-leaves is introduced.

Intel SDM or AMD equivalent "SDM" should not change a CPUID leaf in a way
that it does not consume ECX today but consume it tomorrow.
This is a non-backward compatible instruction change.

With that, it's easy to maintain a list of CPUID leaf that does not have sub-leaf
today, or a list of ones that have sub-leaf.
It's a feature to help on debuggability, similar to today's StrLen() implementation
in BaseLib that avoids reading the memory to no-end.


> 
> >
> > Another concern I have if AsmCpuid() zeros ECX is callers would call
> AsmCpuid(leaf) instead of AsmCpuidEx (leaf, 0).
> > This would cause the caller code a mess.
> 
> I think it's just a mindset that I have from kernel programming. In Linux,
> the AsmCpuid() equivalent will explicitly zero the ECX register so that
> you get the zero sub-leaf if that leaf has sub-leaves.
> 
> Using AsmCpuid() where ECX is non-zero and possibly random, could make
> debugging harder. By zeroing ECX, you will get repeatable return values to
> help with debugging when things aren't going right.

I agree with your concern.
How about having both? I mean not only zeroing out ECX but also rejecting the
CPUID leaves which have sub-leaf.
It's a lib API. I still don't want AsmCpuid to own the responsibility to call sub-leaf 0.
So, in debug build, AsmCpuid() asserts on some calls.
In release build, we get fixed behavior.



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