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Date:   Thu, 05 Mar 2020 18:09:10 -0800
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Subject: Re: [PATCH] KVM: x86: small optimization for is_mtrr_mask calculation
To:     linmiaohe <linmiaohe@huawei.com>,
        Paolo Bonzini <pbonzini@redhat.com>
CC:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>,
        "sean.j.christopherson@intel.com" <sean.j.christopherson@intel.com>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        "jmattson@google.com" <jmattson@google.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>
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On March 5, 2020 6:05:40 PM PST, linmiaohe <linmiaohe@huawei=2Ecom> wrote:
>Hi,
>Paolo Bonzini <pbonzini@redhat=2Ecom> wrote:
>>On 05/03/20 03:48, linmiaohe wrote:
>>> From: Miaohe Lin <linmiaohe@huawei=2Ecom>
>>>=20
>>> We can get is_mtrr_mask by calculating (msr - 0x200) % 2 directly=2E
>>>  		index =3D (msr - 0x200) / 2;
>>> -		is_mtrr_mask =3D msr - 0x200 - 2 * index;
>>> +		is_mtrr_mask =3D (msr - 0x200) % 2;
>>>  		if (!is_mtrr_mask)
>>>  			*pdata =3D vcpu->arch=2Emtrr_state=2Evar_ranges[index]=2Ebase;
>>>  		else
>>>=20
>>
>>If you're going to do that, might as well use ">> 1" for index instead
>of "/ 2", and "msr & 1" for is_mtrr_mask=2E
>>
>
>Many thanks for suggestion=2E What do you mean is like this ?
>
>	index =3D (msr - 0x200) >> 1;
>	is_mtrr_mask =3D msr & 1;
>
>Thanks again=2E

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=2E

Even if it didn't, this code is as far from performance critical as one ca=
n possibly get=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E