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: <38ac916f7c3ae7520708f37389f5524d9278c648.camel@intel.com>
Date: Fri, 17 Oct 2025 20:58:36 +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 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);
>                 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()?

Or we can do things similar to VMX:

        vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",          
                    exit_reason.full);

Or just get rid of this patch :-)