Re: [PATCH v2 2/2] KVM: TDX: WARN if a SEAMCALL VM-Exit makes its way out to KVM

Sean Christopherson
2025-10-17T17:25:25Z

On Fri, Oct 17, 2025, Kai Huang wrote:
> On Thu, 2025-10-16 at 11:21 -0700, Sean Christopherson wrote:
> > WARN if KVM observes a SEAMCALL VM-Exit while running a TD guest, as the
> > TDX-Module is supposed to inject a #UD, per the "Unconditionally Blocked
> > Instructions" section of the TDX-Module base specification.
> >
> > Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > arch/x86/kvm/vmx/tdx.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 097304bf1e1d..ffcfe95f224f 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -2148,6 +2148,9 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
> > * - If it's not an MSMI, no need to do anything here.
> > */
> > return 1;
> > + case EXIT_REASON_SEAMCALL:
> > + WARN_ON_ONCE(1);
> > + break;
> >
>
> While this exit should never happen from a TDX guest, I am wondering why
> we need to explicitly handle the SEAMCALL? E.g., per "Unconditionally
> Blocked Instructions" ENCLS/ENCLV are also listed, therefore
> EXIT_REASON_ELCLS/ENCLV should never come from a TDX guest either.

Good point. SEAMCALL was obviously top of mind, I didn't think about all the
other exits that should be impossible.

I haven't looked closely, at all, but I wonder if we can get away with this?

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 097304bf1e1d..4c68444bd673 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2149,6 +2149,8 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
*/
return 1;
default:
+ /* All other known exits should be handled by the TDX-Module. */
+ WARN_ON_ONCE(exit_reason.basic <= EXIT_REASON_TDCALL);
break;
}