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

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Message-ID: <aPJ8A8u8zIvp-wB4@google.com>
Date: Fri, 17 Oct 2025 10:25:23 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "pbonzini@...hat.com" <pbonzini@...hat.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, Dan J Williams <dan.j.williams@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Rick P Edgecombe <rick.p.edgecombe@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] KVM: TDX: WARN if a SEAMCALL VM-Exit makes its way
 out to KVM

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@...el.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.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;
        }