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

[Paolo Bonzini]

On 31/01/2018 11:50, Jim Mattson wrote:
>> +       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.

In fact this MSR can even be passed down unconditionally, since it needs
no save/restore and has no ill performance effect on the sibling
hyperthread.

Only MSR_IA32_SPEC_CTRL needs to be conditional on
"to_vmx(vcpu)->save_spec_ctrl_on_exit".

Paolo