Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has() callers to boot_cpu_has()

Borislav Petkov
2019-03-31T15:13:00Z

On Sun, Mar 31, 2019 at 04:20:11PM +0200, Paolo Bonzini wrote:
> These are not slow path.

Those functions do a *lot* of stuff like a bunch of MSR reads which are
tens of cycles each at least.

I don't think a RIP-relative MOV and a BT:

movq boot_cpu_data+20(%rip), %rax # MEM[(const long unsigned int *)&boot_cpu_data + 20B], _45
btq $59, %rax #, _45

are at all noticeable.

On latest AMD and Intel uarch those are 2-4 cycles, according to

https://agner.org/optimize/instruction_tables.ods

--
Regards/Gruss,
Boris.

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