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

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Date:   Thu, 5 Mar 2020 15:35:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linmiaohe <linmiaohe@...wei.com>, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, vkuznets@...hat.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] KVM: x86: small optimization for is_mtrr_mask calculation

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.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  arch/x86/kvm/mtrr.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> index 7f0059aa30e1..a98701d9f2bf 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -348,7 +348,7 @@ static void set_var_mtrr_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>  	int index, is_mtrr_mask;
>  
>  	index = (msr - 0x200) / 2;
> -	is_mtrr_mask = msr - 0x200 - 2 * index;
> +	is_mtrr_mask = (msr - 0x200) % 2;
>  	cur = &mtrr_state->var_ranges[index];
>  
>  	/* remove the entry if it's in the list. */
> @@ -424,7 +424,7 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>  		int is_mtrr_mask;
>  
>  		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.

Paolo