KVM: TDX: Handle EXIT_REASON_OTHER_SMI

Handle VM exit caused by "other SMI" for TDX, by returning back to
userspace for Machine Check System Management Interrupt (MSMI) case or
ignoring it and resume vCPU for non-MSMI case.

For VMX, SMM transition can happen in both VMX non-root mode and VMX
root mode.  Unlike VMX, in SEAM root mode (TDX module), all interrupts
are blocked. If an SMI occurs in SEAM non-root mode (TD guest), the SMI
causes VM exit to TDX module, then SEAMRET to KVM. Once it exits to KVM,
SMI is delivered and handled by kernel handler right away.

An SMI can be "I/O SMI" or "other SMI".  For TDX, there will be no I/O SMI
because I/O instructions inside TDX guest trigger #VE and TDX guest needs
to use TDVMCALL to request VMM to do I/O emulation.

For "other SMI", there are two cases:
- MSMI case.  When BIOS eMCA MCE-SMI morphing is enabled, the #MC occurs in
  TDX guest will be delivered as an MSMI.  It causes an
  EXIT_REASON_OTHER_SMI VM exit with MSMI (bit 0) set in the exit
  qualification.  On VM exit, TDX module checks whether the "other SMI" is
  caused by an MSMI or not.  If so, TDX module marks TD as fatal,
  preventing further TD entries, and then completes the TD exit flow to KVM
  with the TDH.VP.ENTER outputs indicating TDX_NON_RECOVERABLE_TD.  After
  TD exit, the MSMI is delivered and eventually handled by the kernel
  machine check handler (7911f14 x86/mce: Implement recovery for
  errors in TDX/SEAM non-root mode), i.e., the memory page is marked as
  poisoned and it won't be freed to the free list when the TDX guest is
  terminated.  Since the TDX guest is dead, follow other non-recoverable
  cases, exit to userspace.
- For non-MSMI case, KVM doesn't need to do anything, just continue TDX
  vCPU execution.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250222014757.897978-17-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/uapi/asm/vmx.h

@@ -34,6 +34,7 @@
 #define EXIT_REASON_TRIPLE_FAULT        2
 #define EXIT_REASON_INIT_SIGNAL			3
 #define EXIT_REASON_SIPI_SIGNAL         4
+#define EXIT_REASON_OTHER_SMI           6
 
 #define EXIT_REASON_INTERRUPT_WINDOW    7
 #define EXIT_REASON_NMI_WINDOW          8

arch/x86/kvm/vmx/tdx.c

@@ -1758,6 +1758,27 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 		return tdx_emulate_io(vcpu);
 	case EXIT_REASON_EPT_MISCONFIG:
 		return tdx_emulate_mmio(vcpu);
+	case EXIT_REASON_OTHER_SMI:
+		/*
+		 * Unlike VMX, SMI in SEAM non-root mode (i.e. when
+		 * TD guest vCPU is running) will cause VM exit to TDX module,
+		 * then SEAMRET to KVM.  Once it exits to KVM, SMI is delivered
+		 * and handled by kernel handler right away.
+		 *
+		 * The Other SMI exit can also be caused by the SEAM non-root
+		 * machine check delivered via Machine Check System Management
+		 * Interrupt (MSMI), but it has already been handled by the
+		 * kernel machine check handler, i.e., the memory page has been
+		 * marked as poisoned and it won't be freed to the free list
+		 * when the TDX guest is terminated (the TDX module marks the
+		 * guest as dead and prevent it from further running when
+		 * machine check happens in SEAM non-root).
+		 *
+		 * - A MSMI will not reach here, it's handled as non_recoverable
+		 *   case above.
+		 * - If it's not an MSMI, no need to do anything here.
+		 */
+		return 1;
 	default:
 		break;
 	}