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SubjectRe: [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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