xen: arm: correctly handle continuations for 64-bit guests

The 64-bit ABI is different to 32-bit:

 - uses x16 as the op register rather than r12.
 - arguments in x0..x5 and not r0..r5. Using rN here potentially
   truncates.
 - return value goes in x0, not r0.

Hypercalls can only be made directly from kernel space, so checking
the domain's size is sufficient.

Spotted due to spurious -EFAULT when destroying a domain, due to the
hypercall's pointer argument being truncated. I'm unclear why I am
only seeing this now.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>

xen/arch/arm/domain.c

@@ -356,29 +356,57 @@ unsigned long hypercall_create_continuation(
     }
     else
     {
-        regs      = guest_cpu_user_regs();
-        regs->r12 = op;
+        regs = guest_cpu_user_regs();
 
         /* Ensure the hypercall trap instruction is re-executed. */
         regs->pc -= 4;  /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
 
-        for ( i = 0; *p != '\0'; i++ )
+#ifdef CONFIG_ARM_64
+        if ( !is_32bit_domain(current->domain) )
         {
-            arg = next_arg(p, args);
+            regs->x16 = op;
 
-            switch ( i )
+            for ( i = 0; *p != '\0'; i++ )
             {
-            case 0: regs->r0 = arg; break;
-            case 1: regs->r1 = arg; break;
-            case 2: regs->r2 = arg; break;
-            case 3: regs->r3 = arg; break;
-            case 4: regs->r4 = arg; break;
-            case 5: regs->r5 = arg; break;
-            }
-        }
+                arg = next_arg(p, args);
 
-        /* Return value gets written back to r0 */
-        rc = regs->r0;
+                switch ( i )
+                {
+                case 0: regs->x0 = arg; break;
+                case 1: regs->x1 = arg; break;
+                case 2: regs->x2 = arg; break;
+                case 3: regs->x3 = arg; break;
+                case 4: regs->x4 = arg; break;
+                case 5: regs->x5 = arg; break;
+                }
+            }
+
+            /* Return value gets written back to x0 */
+            rc = regs->x0;
+        }
+        else
+#endif
+        {
+            regs->r12 = op;
+
+            for ( i = 0; *p != '\0'; i++ )
+            {
+                arg = next_arg(p, args);
+
+                switch ( i )
+                {
+                case 0: regs->r0 = arg; break;
+                case 1: regs->r1 = arg; break;
+                case 2: regs->r2 = arg; break;
+                case 3: regs->r3 = arg; break;
+                case 4: regs->r4 = arg; break;
+                case 5: regs->r5 = arg; break;
+                }
+            }
+
+            /* Return value gets written back to r0 */
+            rc = regs->r0;
+        }
     }
 
     va_end(args);