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

Huang, Kai
2025-10-18T02:48:33Z

On Sat, 2025-10-18 at 09:58 +1300, Kai Huang wrote:
> On Fri, 2025-10-17 at 10:25 -0700, Sean Christopherson wrote:
> > 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 <= c);

Sorry somehow I clobbered the text (at Saturday morning):

WARN_ON_ONCE(exit_reason.basic <= EXIT_REASON_TDCALL);

> > break;
> > }
>
> Not 100% sure, but should be fine? Needs more second eyes here.
>
> E.g., when a new module feature makes another exit reason possible then
> presumably we need explicit opt-in to that feature.
>
> Don't quite follow 'exit_reason.basic <= c' part, though. Maybe we can
> just unconditional WARN_ON_ONCE()?

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