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

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Date:   Fri, 6 Mar 2020 02:05:40 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     "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>, "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH] KVM: x86: small optimization for is_mtrr_mask calculation

Hi,
Paolo Bonzini <pbonzini@...hat.com> wrote:
>On 05/03/20 03:48, linmiaohe wrote:
>> From: Miaohe Lin <linmiaohe@...wei.com>
>> 
>> We can get is_mtrr_mask by calculating (msr - 0x200) % 2 directly.
>>  		index = (msr - 0x200) / 2;
>> -		is_mtrr_mask = msr - 0x200 - 2 * index;
>> +		is_mtrr_mask = (msr - 0x200) % 2;
>>  		if (!is_mtrr_mask)
>>  			*pdata = vcpu->arch.mtrr_state.var_ranges[index].base;
>>  		else
>> 
>
>If you're going to do that, might as well use ">> 1" for index instead of "/ 2", and "msr & 1" for is_mtrr_mask.
>

Many thanks for suggestion. What do you mean is like this ?

	index = (msr - 0x200) >> 1;
	is_mtrr_mask = msr & 1;

Thanks again.