linux-kernel - Re: [PATCH 8/8] KVM: x86: simplify kvm_apic

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Message-ID: <54CBA1F9.6070206@redhat.com>
Date:	Fri, 30 Jan 2015 16:23:37 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Nadav Amit <namit@...technion.ac.il>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH 8/8] KVM: x86: simplify kvm_apic_map



On 30/01/2015 16:14, Radim Krčmář wrote:
> > > +	case KVM_APIC_MODE_XAPIC_FLAT:
> > > +		*cid = 0;
> > > +		*lid = ldr & 0xff;
> > > +		return true;
> > > +	case KVM_APIC_MODE_XAPIC_CLUSTER:
> > > +		*cid = (ldr >> 4) & 0xf;
> > > +		*lid = ldr & 0xf;
> > > +		return true;
> > > +	case KVM_APIC_MODE_X2APIC:
> > > +		*cid = ldr >> 16;
> > > +		*lid = ldr & 0xffff;
> > > +		return true;
> > > +	}
> 
>> > 	lid_bits = mode;
>> > 	cid_bits = mode & (16 | 4);
>> > 	lid_mask = (1 << lid_bits) - 1;
>> > 	cid_mask = (1 << cid_bits) - 1;
>> > 
>> > 	*cid = (ldr >> lid_bits) & cid_mask;
>> > 	*lid = ldr & lid_mask;
> Would jump predictor fail on the switch?  Or is size of the code that
> important?  This code is shorter, but is going to execute far more
> operations, so I think it would be slower ... (And harder to read.)

Considering the additional comparisons for the switch, I don't think
it's going to execute far more operations...

Paolo
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