linux-kernel - Re: [PATCH] KVM: x86: small optimization for is_mtrr

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Message-ID: <CANRm+CwRPDpvN_gcO=VbuukjEL2=OPYnxtqoJO6vTk4Z3nudvQ@mail.gmail.com>
Date:   Fri, 6 Mar 2020 10:27:26 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     "hpa@...or.com" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH] KVM: x86: small optimization for is_mtrr_mask calculation

On Fri, 6 Mar 2020 at 10:23, linmiaohe <linmiaohe@...wei.com> wrote:
>
> hpa@...or.com wrote:
> >>On March 5, 2020 6:05:40 PM PST, linmiaohe <linmiaohe@...wei.com> wrote:
> >>Hi,
> >>Paolo Bonzini <pbonzini@...hat.com> wrote:
> >>Many thanks for suggestion. What do you mean is like this ?
> >>
> >>      index = (msr - 0x200) >> 1;
> >>      is_mtrr_mask = msr & 1;
> >>
> >>Thanks again.
> >
> >You realize that the compiler will probably produce exactly the same code, right? As such, it is about making the code easy for the human reader.
> >
> >Even if it didn't, this code is as far from performance critical as one can possibly get.
>
> Yep, it looks gain little. Thanks.

Please post the performance number when you mention optimize XXX later.

    Wanpeng