Re: [PATCH v3 25/27] KVM: nVMX: Add FRED VMCS fields

[Chao Gao <chao.gao@intel.com>]

>@@ -7197,6 +7250,9 @@ static void nested_vmx_setup_basic(struct nested_vmx_msrs *msrs)
> 	msrs->basic |= VMX_BASIC_TRUE_CTLS;
> 	if (cpu_has_vmx_basic_inout())
> 		msrs->basic |= VMX_BASIC_INOUT;
>+
>+	if (cpu_has_vmx_fred())
>+		msrs->basic |= VMX_BASIC_NESTED_EXCEPTION;

why not advertising VMX_BASIC_NESTED_EXCEPTION if the CPU supports it? just like
VMX_BASIC_INOUT right above.


> }
> 
> static void nested_vmx_setup_cr_fixed(struct nested_vmx_msrs *msrs)
>diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
>index 2c296b6abb8c..5272f617fcef 100644
>--- a/arch/x86/kvm/vmx/nested.h
>+++ b/arch/x86/kvm/vmx/nested.h
>@@ -251,6 +251,14 @@ static inline bool nested_cpu_has_encls_exit(struct vmcs12 *vmcs12)
> 	return nested_cpu_has2(vmcs12, SECONDARY_EXEC_ENCLS_EXITING);
> }
> 
>+static inline bool nested_cpu_has_fred(struct vmcs12 *vmcs12)
>+{
>+	return vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_FRED &&
>+	       vmcs12->vm_exit_controls & VM_EXIT_ACTIVATE_SECONDARY_CONTROLS &&
>+	       vmcs12->secondary_vm_exit_controls & SECONDARY_VM_EXIT_SAVE_IA32_FRED &&
>+	       vmcs12->secondary_vm_exit_controls & SECONDARY_VM_EXIT_LOAD_IA32_FRED;

Is it a requirement in the SDM that the VMM should enable all FRED controls or
none? If not, the VMM is allowed to enable only one or two of them. This means
KVM would need to emulate FRED controls for the L1 VMM as three separate
features.