x86/hvm: add defensive statements in unreachable program points

As a defensive measure, make sure to signal an error to the caller
if an unreachable program point is reached.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/emulate.c

@@ -2673,6 +2673,7 @@ static int _hvm_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt,
 
     default:
         ASSERT_UNREACHABLE();
+        return X86EMUL_UNHANDLEABLE;
     }
 
     if ( hvmemul_ctxt->ctxt.retire.singlestep )

xen/arch/x86/hvm/hvm.c

@@ -4919,6 +4919,8 @@ static int do_altp2m_op(
 
     default:
         ASSERT_UNREACHABLE();
+        rc = -EOPNOTSUPP;
+        break;
     }
 
  out:
@@ -5020,6 +5022,8 @@ static int compat_altp2m_op(
 
     default:
         ASSERT_UNREACHABLE();
+        rc = -EOPNOTSUPP;
+        break;
     }
 
     return rc;

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

@@ -1446,6 +1446,7 @@ struct vmx_msr_entry *vmx_find_msr(const struct vcpu *v, uint32_t msr,
 
     default:
         ASSERT_UNREACHABLE();
+        break;
     }
 
     if ( !start )
@@ -1598,6 +1599,7 @@ int vmx_del_msr(struct vcpu *v, uint32_t msr, enum vmx_msr_list_type type)
 
     default:
         ASSERT_UNREACHABLE();
+        return -EINVAL;
     }
 
     if ( !start )