linux-kernel - Re: [PATCH 10/14] KVM: x86: Make Hyper-V emulation optional

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Date:   Wed, 29 Nov 2023 17:38:52 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/14] KVM: x86: Make Hyper-V emulation optional

On Wed, Oct 25, 2023, Vitaly Kuznetsov wrote:
> @@ -1570,6 +1572,7 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>  	vmcs_load(vmx->loaded_vmcs->vmcs);
>  }
>  
> +#ifdef CONFIG_KVM_HYPERV
>  static void copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx, u32 hv_clean_fields)
>  {
>  	struct vmcs12 *vmcs12 = vmx->nested.cached_vmcs12;
> @@ -2077,6 +2080,10 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
>  
>  	return EVMPTRLD_SUCCEEDED;
>  }
> +#else /* CONFIG_KVM_HYPERV */
> +static inline void copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx, u32 hv_clean_fields) {}
> +static inline void copy_vmcs12_to_enlightened(struct vcpu_vmx *vmx) {}

I'm not sure I love the stubs in .c code.  What if we instead throw the #ifdef
inside the helper, and then add a KVM_BUG_ON() in the CONFIG_KVM_HYPERV=n path?

> +#endif /* CONFIG_KVM_HYPERV */
>  
>  void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
>  {
> @@ -3155,6 +3162,7 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_KVM_HYPERV
>  static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -3182,6 +3190,9 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
>  
>  	return true;
>  }
> +#else
> +static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu) { return true; }
> +#endif

And this one seems like it could be cleaner to just #ifdef the callers.