vm_event: clear up return value of vm_event_monitor_traps

The return value has not been clearly defined, with the function
never returning 0 which seemingly indicated a condition where the
guest should crash.

In this patch we define -rc as error condition where a subscriber is
present but an error prevented the notification from being sent;
0 where there is no subscriber or the notification was sent and the vCPU
is not paused (i.e. safe to continue execution as normal); and 1 where the
notification was sent with the vCPU paused and we are waiting for a
response.

Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

xen/arch/x86/hvm/monitor.c

@@ -48,8 +48,8 @@ bool_t hvm_monitor_cr(unsigned int index, unsigned long value, unsigned long old
             .u.write_ctrlreg.old_value = old
         };
 
-        vm_event_monitor_traps(curr, sync, &req);
-        return 1;
+        if ( vm_event_monitor_traps(curr, sync, &req) >= 0 )
+            return 1;
     }
 
     return 0;

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

@@ -3391,11 +3391,11 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
                 break;
             }
             else {
-                int handled =
+                int rc =
                       hvm_monitor_breakpoint(regs->eip,
                                              HVM_MONITOR_SOFTWARE_BREAKPOINT);
 
-                if ( handled < 0 ) 
+                if ( !rc )
                 {
                     struct hvm_trap trap = {
                         .vector = TRAP_int3,
@@ -3409,7 +3409,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
                     hvm_inject_trap(&trap);
                     break;
                 }
-                else if ( handled )
+                if ( rc > 0 )
                     break;
             }
 

xen/common/vm_event.c

@@ -806,7 +806,7 @@ int vm_event_monitor_traps(struct vcpu *v, uint8_t sync,
          * If there was no ring to handle the event, then
          * simply continue executing normally.
          */
-        return 1;
+        return 0;
     default:
         return rc;
     };
@@ -815,6 +815,7 @@ int vm_event_monitor_traps(struct vcpu *v, uint8_t sync,
     {
         req->flags |= VM_EVENT_FLAG_VCPU_PAUSED;
         vm_event_vcpu_pause(v);
+        rc = 1;
     }
 
     if ( altp2m_active(d) )
@@ -826,7 +827,7 @@ int vm_event_monitor_traps(struct vcpu *v, uint8_t sync,
     vm_event_fill_regs(req);
     vm_event_put_request(d, &d->vm_event->monitor, req);
 
-    return 1;
+    return rc;
 }
 
 /*