Re: [PATCH v2 1/2] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2025-10-17 at 06:00 -0700, Sean Christopherson wrote:
> On Fri, Oct 17, 2025, Kai Huang wrote:
> > 
> > > --- a/arch/x86/kvm/vmx/nested.c
> > > +++ b/arch/x86/kvm/vmx/nested.c
> > > @@ -6728,6 +6728,14 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu,
> > >  	case EXIT_REASON_NOTIFY:
> > >  		/* Notify VM exit is not exposed to L1 */
> > >  		return false;
> > > +	case EXIT_REASON_SEAMCALL:
> > > +	case EXIT_REASON_TDCALL:
> > > +		/*
> > > +		 * SEAMCALL and TDCALL unconditionally VM-Exit, but aren't
> > > +		 * virtualized by KVM for L1 hypervisors, i.e. L1 should
> > > +		 * never want or expect such an exit.
> > > +		 */
> > > +		return false;
> > 
> > Sorry for commenting late.
> > 
> > I think from emulating hardware behaviour's perspective, if L1 doesn't
> > support TDX (obviously true), SEAMCALL/TDCALL in L2 should cause VMEXIT to
> > L1.  In other words, L1 is expecting a VMEXIT in such case.
> 
> No, because from L1's perspective, the opcodes map to undefined instructions and
> thus should #UD in L2.  There's no super explicit enumeration, but IMO it's fair
> to say that for L1 to think the instructions exists, it would need to observe
> IA32_SEAMRR_PHYS_{BASE,MASK} for SEAMCALL, and MSR_IA32_MKTME_KEYID_PARTITIONING
> as well for TDCALL.  KVM doesn't emulate any of those instructions, and so L1
> should never expect SEAMCALL or TDCALL to do anything other than #UD.
> 

Oh right.  I forgot the SEAMCALL/TDCALL VMEXIT only happens on TDX-capable
machine.  :-)
>