vmx: Handle TDX instruction exit reasons

On hardware that supports TDX, guests can cause VMEXIT related to
TDX instructions (SEAMCALL and TDCALL) regardless of VMCS configuration.

Currently, that causes the domain to crash when a domain tries to
use these instructions in kernel-mode, and emit #UD when used in user-mode.

Adjust the behavior so that the guest always gets #UD when trying to
use these instructions regardless of the privilege level that the vCPU
is running.

In the nested virtualization case, also reinject the exit reason to L1
rather than failing on "Unhandled nested vmexit" (leading to a L1 crash
that can be caused by L2 by executing one of the TDX instructions).

Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 755c9fbbfe
master date: 2026-09-22 09:59:37 +0200

xen/arch/x86/hvm/vmx/vmx.c

@@ -4681,6 +4681,8 @@ void asmlinkage vmx_vmexit_handler(struct cpu_user_regs *regs)
     case EXIT_REASON_MWAIT_INSTRUCTION:
     case EXIT_REASON_MONITOR_INSTRUCTION:
     case EXIT_REASON_GETSEC:
+    case EXIT_REASON_SEAMCALL:
+    case EXIT_REASON_TDCALL:
         /*
          * We should never exit on GETSEC because CR4.SMXE is always 0 when
          * running in guest context, and the CPU checks that before getting

xen/arch/x86/hvm/vmx/vvmx.c

@@ -2496,6 +2496,8 @@ int nvmx_n2_vmexit_handler(struct cpu_user_regs *regs,
     case EXIT_REASON_INVEPT:
     case EXIT_REASON_XSETBV:
     case EXIT_REASON_INVVPID:
+    case EXIT_REASON_SEAMCALL:
+    case EXIT_REASON_TDCALL:
         /* inject to L1 */
         nvcpu->nv_vmexit_pending = 1;
         break;

xen/arch/x86/include/asm/hvm/vmx/vmx.h

@@ -201,6 +201,8 @@ static inline void pi_clear_sn(struct pi_desc *pi_desc)
 #define EXIT_REASON_XRSTORS             64
 #define EXIT_REASON_BUS_LOCK            74
 #define EXIT_REASON_NOTIFY              75
+#define EXIT_REASON_SEAMCALL            76
+#define EXIT_REASON_TDCALL              77
 /* Remember to also update VMX_PERF_EXIT_REASON_SIZE! */
 
 /*

xen/arch/x86/include/asm/perfc_defn.h

@@ -6,7 +6,7 @@ PERFCOUNTER_ARRAY(exceptions,           "exceptions", 32)
 
 #ifdef CONFIG_HVM
 
-#define VMX_PERF_EXIT_REASON_SIZE 76
+#define VMX_PERF_EXIT_REASON_SIZE 78
 #define VMEXIT_NPF_PERFC 166
 #define SVM_PERF_EXIT_REASON_SIZE (VMEXIT_NPF_PERFC + 1)
 PERFCOUNTER_ARRAY(vmexits,              "vmexits",