xen/arm: fix unmapped access trapping on GICv2 hardware

Since 4dbcb06536, the vGICv2 CPU interface is mapped in a deferred
manner. On domains with XEN_DOMCTL_CDF_trap_unmapped_accesses unset on
GICv2 hardware, the vGICv2 CPU interface fails to be mapped. A visible
symptom is that a domU gets stuck at:

  [    0.177983] smp: Bringing up secondary CPUs ...

Move the 2nd check_p2m earlier so it's prioritized over try_handle_mmio.

Fixes: 980aff4e8f ("xen/arm: Add way to disable traps on accesses to unmapped addresses")
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>

xen/arch/arm/traps.c

@@ -1915,6 +1915,14 @@ static void do_trap_stage2_abort_guest(struct cpu_user_regs *regs,
         if ( info.dabt_instr.state == INSTR_ERROR )
             goto inject_abt;
 
+        /*
+         * If the instruction syndrome was invalid, then we already checked if
+         * this was due to a P2M fault. So no point to check again as the result
+         * will be the same.
+         */
+        if ( (info.dabt_instr.state == INSTR_VALID) && check_p2m(is_data, gpa) )
+            return;
+
         state = try_handle_mmio(regs, &info);
 
         switch ( state )
@@ -1939,14 +1947,6 @@ static void do_trap_stage2_abort_guest(struct cpu_user_regs *regs,
                 break;
         }
 
-        /*
-         * If the instruction syndrome was invalid, then we already checked if
-         * this was due to a P2M fault. So no point to check again as the result
-         * will be the same.
-         */
-        if ( (info.dabt_instr.state == INSTR_VALID) && check_p2m(is_data, gpa) )
-            return;
-
         break;
     }
     default: