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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 6/13/22 18:16, Anirudh Rayabharam wrote:
>> +	if (!kvm_has_tsc_control)
>> +		msrs->secondary_ctls_high &= ~SECONDARY_EXEC_TSC_SCALING;
>> +
>>   	msrs->secondary_ctls_low = 0;
>>   	msrs->secondary_ctls_high &=
>>   		SECONDARY_EXEC_DESC |
>> @@ -6667,8 +6670,7 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>   		SECONDARY_EXEC_RDRAND_EXITING |
>>   		SECONDARY_EXEC_ENABLE_INVPCID |
>>   		SECONDARY_EXEC_RDSEED_EXITING |
>> -		SECONDARY_EXEC_XSAVES |
>> -		SECONDARY_EXEC_TSC_SCALING;
>> +		SECONDARY_EXEC_XSAVES;
>>   
>>   	/*
>
> This is wrong because it _always_ disables SECONDARY_EXEC_TSC_SCALING,
> even if kvm_has_tsc_control == true.
>
> That said, I think a better implementation of this patch is to just add
> a version of evmcs_sanitize_exec_ctrls that takes a struct
> nested_vmx_msrs *, and call it at the end of nested_vmx_setup_ctl_msrs like
>
> 	evmcs_sanitize_nested_vmx_vsrs(msrs);
>
> Even better (but I cannot "mentally test it" offhand) would be just
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e802f71a9e8d..b3425ce835c5 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1862,7 +1862,7 @@ int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		 * sanity checking and refuse to boot. Filter all unsupported
>   		 * features out.
>   		 */
> -		if (!msr_info->host_initiated &&
> +		if (static_branch_unlikely(&enable_evmcs) ||
>   		    vmx->nested.enlightened_vmcs_enabled)
>   			nested_evmcs_filter_control_msr(msr_info->index,
>   							&msr_info->data);
>
> I cannot quite understand the host_initiated check, so I'll defer to
> Vitaly on why it is needed. Most likely, removing it would cause some
> warnings in QEMU with e.g. "-cpu Haswell,+vmx"; but I think it's a
> userspace bug and we should remove that part of the condition. 

I forgot the details, of course, but 31de3d2500e4 says:

```
    With fine grained VMX feature enablement QEMU>=4.2 tries to do KVM_SET_MSRS
    with default (matching CPU model) values and in case eVMCS is also enabled,
    fails.
```

so it certainly was a workaround for QEMU.

-- 
Vitaly