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

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Message-ID: <2129995e-3441-f362-aed1-7c247189c136@redhat.com>
Date:   Thu, 5 Mar 2020 16:25:20 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Laight <David.Laight@...LAB.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

On 05/03/20 16:10, David Laight wrote:
>>>  	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.

Yes, I'm old-fashioned, but also I prefer ">>" and "&" for both signed
and unsigned, because if ever I need to switch from unsigned to signed I
will get floor-division instead of round-to-zero division (most likely
the code doesn't expect negative remainders if it was using unsigned).

(That perhaps also reflects on me working a lot with Smalltalk long
before switching to the kernel...).

Paolo