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SubjectRe: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch

On 2019/11/30 14:39, Marc Zyngier wrote:
> On Sat, 30 Nov 2019 03:14:43 +0000,
> Heyi Guo <> wrote:
>> Platforms running hypervisor nowadays are normally powerful servers
>> which at least support GICv3, so it should be better to switch
>> kvm_vgic_global_state.gicv3_cpuif to static likely branch, which can
>> reduce two "b" instructions to a single "nop" for GICv3 branches.
>> We don't update arm32 specific code for they may still only have
>> GICv2.
> There is a number of disputable statements here.
> Out of the fairly large zoo of arm64 systems I have access to, 75% of
> them are based on GICv2, so they are still the overwhelming majority.
> Yes, they all run KVM (otherwise I would ignore them).
Really? I'm surprised to know that... Sorry I didn't see such GICv2
platforms in my work, so I made the wrong assumption.
I don't expect much performance improvement for GICv3 platforms. The
precondition for this patch is that few platforms running KVM are using
GICv2. If it is not right, please just ignore it.


> Furthermore, I would expect that "powerful servers" are perfectly
> capable to execute a couple of branches without breaking a sweat.
> Finally, you don't provide any number supporting that:
> - GICv3 systems see a performance improvement across the large variety
> of CPU implementations
> - GICv2 systems don't see a performance regression
> Once you provide such numbers, I'll reevaluate my position. Until
> then, I'm not considering this kind of change.
> Thanks,
> M.

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