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: <3e16e69ce147db64abfc52464cc3be0358f11ae8.camel@intel.com>
Date: Fri, 17 Oct 2025 21:11:12 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Edgecombe, Rick P"
	<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 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@...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 <= 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()?

... and copied here..