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List:       kvm
Subject:    Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has() callers to boot_cpu_has()
From:       Borislav Petkov <bp () alien8 ! de>
Date:       2019-03-31 15:12:48
Message-ID: 20190331151248.GB15073 () zn ! tnic
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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.

Good mailing practices for 400: avoid top-posting and trim the reply.


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