linux-kernel - Re: [PATCH v3 1/4] KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

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Date:   Fri, 5 Nov 2021 00:52:14 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

On Wed, Oct 13, 2021, Vitaly Kuznetsov wrote:
> 3-level nesting is also not a very common setup nowadays.

Says who? :-D

> Don't enable 'Enlightened MSR Bitmap' feature for KVM's L2s (real L3s) for
> now.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1c8b2b6e7ed9..e82cdde58119 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2655,15 +2655,6 @@ int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>  		if (!loaded_vmcs->msr_bitmap)
>  			goto out_vmcs;
>  		memset(loaded_vmcs->msr_bitmap, 0xff, PAGE_SIZE);
> -
> -		if (IS_ENABLED(CONFIG_HYPERV) &&
> -		    static_branch_unlikely(&enable_evmcs) &&
> -		    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
> -			struct hv_enlightened_vmcs *evmcs =
> -				(struct hv_enlightened_vmcs *)loaded_vmcs->vmcs;
> -
> -			evmcs->hv_enlightenments_control.msr_bitmap = 1;
> -		}
>  	}
>  
>  	memset(&loaded_vmcs->host_state, 0, sizeof(struct vmcs_host_state));
> @@ -6903,6 +6894,18 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
>  
>  	vmx->loaded_vmcs = &vmx->vmcs01;
>  
> +	/*
> +	 * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a
> +	 * nested (L1) hypervisor and Hyper-V in L0 supports it.

And maybe call out specifically that KVM intentionally uses this only for vmcs02?

> +	 */
> +	if (IS_ENABLED(CONFIG_HYPERV) && static_branch_unlikely(&enable_evmcs)
> +	    && (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {

&& on the previous line, I think we'll survive the 82 char line :-)

> +		struct hv_enlightened_vmcs *evmcs =
> +			(struct hv_enlightened_vmcs *)vmx->loaded_vmcs->vmcs;

Hmm, what about landing this right after vmcs01's VMCS is allocated?  It's kinda
weird, but it makes it more obvious that ->vmcs is not NULL.  And if the cast is
simply via a "void *" it all fits on one line.

	err = alloc_loaded_vmcs(&vmx->vmcs01);
	if (err < 0)
		goto free_pml;

	/*
	 * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a
	 * nested (L1) hypervisor and Hyper-V in L0 supports it.  Enable an
	 * enlightened bitmap only for vmcs01, KVM currently isn't equipped to
	 * realize any performance benefits from enabling it for vmcs02.
	 */ 
	if (IS_ENABLED(CONFIG_HYPERV) && static_branch_unlikely(&enable_evmcs) &&
	    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
		struct hv_enlightened_vmcs *evmcs = (void *)vmx->vmcs01.vmcs;

		evmcs->hv_enlightenments_control.msr_bitmap = 1;
	}

> +
> +		evmcs->hv_enlightenments_control.msr_bitmap = 1;
> +	}
> +
>  	if (cpu_need_virtualize_apic_accesses(vcpu)) {
>  		err = alloc_apic_access_page(vcpu->kvm);
>  		if (err)
> -- 
> 2.31.1
>