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Date:   Sun, 31 Mar 2019 17:12:48 +0200
From:   Borislav Petkov <>
To:     Paolo Bonzini <>
Cc:     LKML <>,
        Nadav Amit <>,
        Andy Lutomirski <>,
        Peter Zijlstra <>,
        "H. Peter Anvin" <>, Ingo Molnar <>,
        Joerg Roedel <>,,
        Radim Krčmář <>,
        Thomas Gleixner <>,
Subject: Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has()
 callers to boot_cpu_has()

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


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