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

[KarimAllah Ahmed <karahmed@amazon.com>]

On 01/31/2018 05:55 PM, Paolo Bonzini wrote:
> 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".

That used to be the case in an earlier version. There seems to be two
opinions here:

1) Pass it only if CPUID for the guest has it.
2) Pass it unconditionally.

I do not really have a preference.


> 
> Paolo
> 
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