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

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Date:   Thu, 5 Mar 2020 15:10:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Paolo Bonzini' <pbonzini@...hat.com>,
        linmiaohe <linmiaohe@...wei.com>,
        "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>
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>
Subject: RE: [PATCH] KVM: x86: small optimization for is_mtrr_mask calculation

From: Paolo Bonzini
> Sent: 05 March 2020 14:36
> 
> 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.

Provided the variables are unsigned it makes little difference
whether you use / % or >> &.
At least with / % the two values are the same.

	David

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