Re: [PATCH] KVM: nVMX: Don't expose TSC scaling to L1 when on Hyper-V

[Paolo Bonzini <pbonzini@redhat.com>]

On 6/14/22 17:13, Anirudh Rayabharam wrote:
>>> Sanitize at the end might not work because I see some cases in
>>> nested_vmx_setup_ctls_msrs() where we want to expose some things to L1
>>> even though the hardware doesn't support it.
>>
>> Yes, but these will never include eVMCS-unsupported features.
>
> How are you so sure?
> 
> For example, SECONDARY_EXEC_SHADOW_VMCS is unsupported in eVMCS but in
> nested_vmx_setup_ctls_msrs() we do:
> 
> 6675         /*
> 6676          * We can emulate "VMCS shadowing," even if the hardware
> 6677          * doesn't support it.
> 6678          */
> 6679         msrs->secondary_ctls_high |=
> 6680                 SECONDARY_EXEC_SHADOW_VMCS;
> 
> If we sanitize this out it might cause some regression right?

Yes, you're right, shadow VMCS is special: it is not supported by 
enlightened VMCS, but it is emulated rather than virtualized. 
Therefore, if L1 does not use the enlightened VMCS, it can indeed use 
shadow VMCS.

Paolo