Re: [PATCH] x86/hyper-v: Don't WARN on VM-Enter if eVMCS sees VMFUNC[63:32] != 0

[Manuel Andreas <manuel.andreas@tum.de>]

On 10/21/26 00:56, Sean Christopherson wrote:
> I think the WARN is correct, this code is broken.  Unless I'm missing some eVMCS
> magic/quirk, this, and the secondary controls check, should be conditioned on the
> field actually being used.
> 
> This, over two patches?
> 
> diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
> index fa41d036acd4..a1daf8f2ec5a 100644
> --- a/arch/x86/kvm/vmx/hyperv.c
> +++ b/arch/x86/kvm/vmx/hyperv.c
> @@ -169,7 +169,8 @@ int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
>  					       vmcs12->cpu_based_vm_exec_control)))
>  		return -EINVAL;
>  
> -	if (CC(!nested_evmcs_is_valid_controls(EVMCS_2NDEXEC,
> +	if (nested_cpu_has(vmcs12, CPU_BASED_ACTIVATE_SECONDARY_CONTROLS) &&
> +	    CC(!nested_evmcs_is_valid_controls(EVMCS_2NDEXEC,
>  					       vmcs12->secondary_vm_exec_control)))
>  		return -EINVAL;
>  
> @@ -186,11 +187,10 @@ int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
>  	 * controls in bits 63:32, i.e. dropping those bits on the consistency
>  	 * check is intentional.
>  	 */
> -	if (WARN_ON_ONCE(vmcs12->vm_function_control >> 32))
> -		return -EINVAL;
> -
> -	if (CC(!nested_evmcs_is_valid_controls(EVMCS_VMFUNC,
> -					       vmcs12->vm_function_control)))
> +	if (nested_cpu_has_vmfunc(vmcs12) &&
> +	    (WARN_ON_ONCE(vmcs12->vm_function_control >> 32) ||
> +	     CC(!nested_evmcs_is_valid_controls(EVMCS_VMFUNC,
> +						vmcs12->vm_function_control))))
>  		return -EINVAL;
>  
>  	return 0;
> 
> 
Looks sane to me. However, I still don't see a reason for this being a
WARN at all, what exactly is the intended signal here?

Best,
Manuel