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    SubjectRe: [PATCH v4 2/5] KVM: x86: Add IBPB support
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    On 31/01/2018 12:11, KarimAllah Ahmed wrote:
    > On 01/31/2018 05:50 PM, Jim Mattson wrote:
    >> On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@amazon.de>
    >> 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.

    Sorry, that was for spec_ctrl. Here there's no need to do any kind of
    conditional check.

    Paolo

    >> Also, the final argument to
    >> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
    >> MSR_TYPE_R.
    >>
    > Oops! will fix!
    > Amazon Development Center Germany GmbH
    > Berlin - Dresden - Aachen
    > main office: Krausenstr. 38, 10117 Berlin
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