CODEWITHSUNDEEP
x86x86-64cpuid
user6562673
user6562673

Reputation:

Max CPUID standard leaf / CPUID-features dependent on MSRs?

Is the maximum CPUID standard leaf changeable, i.e. might it be reconfigured by the kernel to enable additional CPUID values ?

Are there CPUID feature-bits that are dependent on the configuration of certain MSRs ?

Upvotes: 0

Views: 316

Answers (2)

prl
prl

Reputation: 12455

There are several CPUID results that can be affected by control registers or MSRs set by the OS.

leaf (subleaf) register[bit] controlled by
7 (0) ECX[4] OSPKE CR4.PKE
9 EAX[31:0] IA32_PLATFORM_DCA_CAP
D (0) EBX[31:0] XCR0
D (1) EBX[31:0] XCR0 and IA32_XSS
80000001 EDX[11] SYSCALL/SYSRET IA32_EFER.LMA

Upvotes: 1

Brendan
Brendan

Reputation: 37222

Is the maximum CPUID standard leaf changeable, i.e. might it be reconfigured by the kernel to enable additional CPUID values ?

In general, no (the max. standard leaf reported is the literal maximum), however...

A very long time ago a Windows NT kernel was buggy and crashed if the CPU reported a "too high" max. CPUID value (which wasn't detected when the kernel was released because CPUs didn't report as much back then); so to fix the problem Intel said "Software is soft, hardware is hard, let's add a hack to hardware for the next 20+ years so Microsoft doesn't need to write a 2 minute patch". The end result is a "Limit CPUID MAXVAL" flag at bit 22 of the IA32_MISC_ENABLE MSR (MSR 0x1A0); which (if set) will cause CPUID to report a max. value of 3 (and not the real, higher, max value).

For other 80x86 CPU manufacturers (AMD, VIA, Cyrix, NexGen, ..) they didn't mess with the max. standard CPUID leaf themselves (and created their own areas - e.g. the "0x80000000+ leafs" used by AMD and copied Intel for the standard leaves where they can); but a lot of them allow things like CPU vendor string or CPU brand string or CPU features flags to be modified so you can't trust them much (e.g. it wouldn't be hard to modify a boot loader to make old AMD Athlon chips report "Core i9 8765 @ 6 GHz" and sell the computers 2nd hand).

Upvotes: 2

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