linux-kernel - Re: [RFC PATCH 17/18] KVM: VMX: Allow MBEC with EVMCS

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Message-ID: <aCJphDlQLyWri9kR@google.com>
Date: Mon, 12 May 2025 14:35:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jon Kohler <jon@...anix.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [RFC PATCH 17/18] KVM: VMX: Allow MBEC with EVMCS

On Thu, Mar 13, 2025, Jon Kohler wrote:
> Extend EVMCS1_SUPPORTED_2NDEXEC to understand MBEC enablement,
> otherwise presenting both EVMCS and MBEC at the same time will disable
> MBEC presentation into the guest.

A brief rundown on any relevant history of eVMCS support for MBEC would be
appreciated, if there is any.
 
> Signed-off-by: Jon Kohler <jon@...anix.com>
> 
> ---
>  arch/x86/kvm/vmx/hyperv.c       | 5 ++++-
>  arch/x86/kvm/vmx/hyperv_evmcs.h | 1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
> index fab6a1ad98dc..941a29c9e667 100644
> --- a/arch/x86/kvm/vmx/hyperv.c
> +++ b/arch/x86/kvm/vmx/hyperv.c
> @@ -138,7 +138,10 @@ void nested_evmcs_filter_control_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *
>  		ctl_high &= evmcs_get_supported_ctls(EVMCS_EXEC_CTRL);
>  		break;
>  	case MSR_IA32_VMX_PROCBASED_CTLS2:
> -		ctl_high &= evmcs_get_supported_ctls(EVMCS_2NDEXEC);
> +		supported_ctrls = evmcs_get_supported_ctls(EVMCS_2NDEXEC);
> +		if (!vcpu->arch.pt_guest_exec_control)
> +			supported_ctrls &= ~SECONDARY_EXEC_MODE_BASED_EPT_EXEC;

No idea what you're trying to do, but I don't see how this is necessary in any
capacity.

> +		ctl_high &= supported_ctrls;
>  		break;
>  	case MSR_IA32_VMX_TRUE_PINBASED_CTLS:
>  	case MSR_IA32_VMX_PINBASED_CTLS:
> diff --git a/arch/x86/kvm/vmx/hyperv_evmcs.h b/arch/x86/kvm/vmx/hyperv_evmcs.h
> index a543fccfc574..930429f376f9 100644
> --- a/arch/x86/kvm/vmx/hyperv_evmcs.h
> +++ b/arch/x86/kvm/vmx/hyperv_evmcs.h
> @@ -87,6 +87,7 @@
>  	 SECONDARY_EXEC_PT_CONCEAL_VMX |				\
>  	 SECONDARY_EXEC_BUS_LOCK_DETECTION |				\
>  	 SECONDARY_EXEC_NOTIFY_VM_EXITING |				\
> +	 SECONDARY_EXEC_MODE_BASED_EPT_EXEC |				\
>  	 SECONDARY_EXEC_ENCLS_EXITING)
>  
>  #define EVMCS1_SUPPORTED_3RDEXEC (0ULL)
> -- 
> 2.43.0
>