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

[KarimAllah Ahmed]

On 01/31/2018 05:50 PM, Jim Mattson wrote:
> 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)"?

Paolo suggested this on the previous revision because guest_cpuid_has()
would be slow.

> Also, the final argument to
> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
> MSR_TYPE_R.
Oops! will fix!
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