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

[Anirudh Rayabharam]

On Mon, Jun 13, 2022 at 06:49:17PM +0200, Paolo Bonzini wrote:
> 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.

The MSR actually allows 1-setting of the "use TSC scaling" control. So this
line is redundant anyway.

> 
> 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);

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.

> 
> 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 will try this.

Thanks,

	Anirudh.

> 
> 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.  You
> don't need to worry about that part, we'll cross that bridge if the
> above patch works for your case.
> 
> Thanks,
> 
> Paolo