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SubjectRe: [PATCH] KVM: nVMX: nested TPR shadow/threshold emulation
Il 01/08/2014 02:57, Zhang, Yang Z ha scritto:
> > TPR_THRESHOLD will be likely written as zero, but the processor will
> > never use it anyway. It's just a small optimization because
> > nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW) will almost always be true.
>
> Theoretically, you are right. But we should not expect all VMMs
> follow it. It is not worth to violate the SDM just for saving two or
> three instructions' cost.

Yes, you do need an "if (cpu_has_vmx_tpr_shadow())" around the
vmcs_write32. But still, checking nested_cpu_has is not strictly
necessary. Right now they both are a single AND, but I have plans to
change all of the cpu_has_*() checks to static keys.

Paolo


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