Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

[Jim Mattson]

On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@xxxxxxxxx> wrote:

> +               vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
> +                                             MSR_TYPE_W);

Why not disable this intercept eagerly, rather than lazily? Unlike
MSR_IA32_SPEC_CTRL, there is no guest value to save/restore, so there
is no cost to disabling the intercept if the guest cpuid info declares
support for it.


> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
> +               nested_vmx_disable_intercept_for_msr(
> +                               msr_bitmap_l1, msr_bitmap_l0,
> +                               MSR_IA32_PRED_CMD,
> +                               MSR_TYPE_R);
> +       }

I don't think this should be predicated on
"to_vmx(vcpu)->save_spec_ctrl_on_exit." Why not just
"guest_cpuid_has(vcpu, X86_FEATURE_IBPB)"? Also, the final argument to
nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
MSR_TYPE_R.