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

[Sean Christopherson <seanjc@google.com>]

On Wed, Oct 21, 2026, Manuel Andreas wrote:
> 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?

That normal, non-eVMCS VMX added support for a VMFUNC operation that lives in
bits 63:32 and thus nested_evmcs_is_valid_controls() needs to be updated to play
nice with 64-bit values.