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

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Message-ID: <20190331151248.GB15073@zn.tnic>
Date:   Sun, 31 Mar 2019 17:12:48 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
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

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

-- 
Regards/Gruss,
    Boris.

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