arm: use r12 to pass the hypercall number

** This is a guest visible ABI change which requires an updated guest kernel **

Use r12 to pass the hypercall number and r0-r4 for the hypercall
arguments.

Use the ISS to pass an hypervisor specific tag.

Remove passing unused registers to arm_hypercall_table: we don't have 6
arguments hypercalls and we never use 64 bit values as hypercall
arguments, 64 bit values are only contained within structs passed as
arguments.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[ use #ifndef NDEBUG, fix coding style, expand calling convention comment
  slightly and added a big fat note about ABI change - ijc ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/traps.c

@@ -367,7 +367,6 @@ unsigned long do_arch_0(unsigned int cmd, unsigned long long value)
 }
 
 typedef unsigned long arm_hypercall_t(
-    unsigned int, unsigned int, unsigned int, unsigned int, unsigned int,
     unsigned int, unsigned int, unsigned int, unsigned int, unsigned int);
 
 #define HYPERCALL(x)                                        \
@@ -407,18 +406,30 @@ static void do_debug_trap(struct cpu_user_regs *regs, unsigned int code)
 
 static void do_trap_hypercall(struct cpu_user_regs *regs, unsigned long iss)
 {
+    arm_hypercall_t *call = NULL;
     local_irq_enable();
 
-    regs->r0 = arm_hypercall_table[iss](regs->r0,
-                             regs->r1,
-                             regs->r2,
-                             regs->r3,
-                             regs->r4,
-                             regs->r5,
-                             regs->r6,
-                             regs->r7,
-                             regs->r8,
-                             regs->r9);
+    if ( iss != XEN_HYPERCALL_TAG )
+    {
+        printk("%s %d: received an alien hypercall iss=%lx\n", __func__ ,
+                __LINE__ , iss);
+        regs->r0 = -EINVAL;
+        return;
+    }
+
+    call = arm_hypercall_table[regs->r12];
+    if ( call == NULL )
+    {
+        regs->r0 = -ENOSYS;
+        return;
+    }
+
+    regs->r0 = call(regs->r0, regs->r1, regs->r2, regs->r3, regs->r4);
+
+#ifndef NDEBUG
+    /* clobber registers */
+    regs->r1 = regs->r2 = regs->r3 = regs->r4 = regs->r12 = 0xDEADBEEF;
+#endif
 }
 
 static void do_cp15_32(struct cpu_user_regs *regs,

xen/include/public/arch-arm.h

@@ -27,6 +27,29 @@
 #ifndef __XEN_PUBLIC_ARCH_ARM_H__
 #define __XEN_PUBLIC_ARCH_ARM_H__
 
+/* hypercall calling convention
+ * ----------------------------
+ *
+ * A hypercall is issued using the ARM HVC instruction.
+ *
+ * A hypercall can take up to 5 arguments. These are passed in
+ * registers, the first argument in r0, the second argument in r1, the
+ * third in r2, the forth in r3 and the fifth in r4.
+ *
+ * The hypercall number is passed in r12.
+ *
+ * The HVC ISS must contain a Xen specific TAG: XEN_HYPERCALL_TAG.
+ *
+ * The return value is in r0.
+ *
+ * The hypercall will always clobber r0, r1, r2, r3, r4 and r12,
+ * regardless of how many arguments the particular hypercall takes.
+ *
+ */
+
+#define XEN_HYPERCALL_TAG   0XEA1
+
+
 #ifndef __ASSEMBLY__
 #define ___DEFINE_XEN_GUEST_HANDLE(name, type) \
     typedef struct { type *p; } __guest_handle_ ## name