Re: x86 CPUID mutable return values

[Dave Hansen]

On 9/21/26 11:36, Christian Ludloff wrote:
> Q for Intel: is MSR FEATURE_CONFIG 0x13C bit 1 expected
> to affect CPUID 1.ECX.25 and 7.0.ECX.9 and 19.EBX.0/2, or
> just just a subset of them? Also, what about PCLMUL?

Are you looking to document architecture or implementation here?

I poked around on a few pieces of hardware I have within reach and all
of them have the MSR FEATURE_CONFIG lock bit set and the disable bit clear.

# rdmsr -a 0x13C
1
1
1
1
...

In practice, I suspect the AESNI CPUID bits are immutable.

FEATURE_CONFIG's definitions also looks model specific in the spots I
see it in the SDM. They're also not identical, interestingly enough.

Honestly, FEATURE_CONFIG looks more like something that should have been
in the BIOS writers' guide rather than the SDM.