linux-kernel - Re: [PATCH v3 1/3] KVM: nVMX: Sync L2 guest CET states between L1/L2

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Date:   Thu, 4 Mar 2021 08:46:45 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] KVM: nVMX: Sync L2 guest CET states between L1/L2

On Thu, Mar 04, 2021, Yang Weijiang wrote:
> @@ -3375,6 +3391,12 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
>  	if (kvm_mpx_supported() &&
>  		!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS))
>  		vmx->nested.vmcs01_guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
> +	if (kvm_cet_supported() &&
> +		!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {

Alignment.

> +		vmx->nested.vmcs01_guest_ssp = vmcs_readl(GUEST_SSP);
> +		vmx->nested.vmcs01_guest_s_cet = vmcs_readl(GUEST_S_CET);
> +		vmx->nested.vmcs01_guest_ssp_tbl = vmcs_readl(GUEST_INTR_SSP_TABLE);
> +	}
>  
>  	/*
>  	 * Overwrite vmcs01.GUEST_CR3 with L1's CR3 if EPT is disabled *and*
> @@ -4001,6 +4023,9 @@ static bool is_vmcs12_ext_field(unsigned long field)
>  	case GUEST_IDTR_BASE:
>  	case GUEST_PENDING_DBG_EXCEPTIONS:
>  	case GUEST_BNDCFGS:
> +	case GUEST_SSP:
> +	case GUEST_INTR_SSP_TABLE:
> +	case GUEST_S_CET:
>  		return true;
>  	default:
>  		break;
> @@ -4052,6 +4077,11 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
>  		vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
>  	if (kvm_mpx_supported())
>  		vmcs12->guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
> +	if (kvm_cet_supported()) {

Isn't the existing kvm_mpx_supported() check wrong in the sense that KVM only
needs to sync to vmcs12 if KVM and the guest both support MPX?  Same would
apply to CET.  Not sure it'd be a net positive in terms of performance since
guest_cpuid_has() can be quite slow, but overwriting vmcs12 fields that
technically don't exist feels wrong.

> +		vmcs12->guest_ssp = vmcs_readl(GUEST_SSP);
> +		vmcs12->guest_s_cet = vmcs_readl(GUEST_S_CET);
> +		vmcs12->guest_ssp_tbl = vmcs_readl(GUEST_INTR_SSP_TABLE);
> +	}
>  
>  	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;
>  }
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 9d3a557949ac..36dc4fdb0909 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -155,6 +155,9 @@ struct nested_vmx {
>  	/* to migrate it to L2 if VM_ENTRY_LOAD_DEBUG_CONTROLS is off */
>  	u64 vmcs01_debugctl;
>  	u64 vmcs01_guest_bndcfgs;
> +	u64 vmcs01_guest_ssp;
> +	u64 vmcs01_guest_s_cet;
> +	u64 vmcs01_guest_ssp_tbl;
>  
>  	/* to migrate it to L1 if L2 writes to L1's CR8 directly */
>  	int l1_tpr_threshold;
> -- 
> 2.26.2
>