xen/arm64: head: Rework and document launch()

Boot CPU and secondary CPUs will use different entry point to C code. At
the moment, the decision on which entry to use is taken within launch().

In order to avoid a branch for the decision and make the code clearer,
launch() is reworked to take in parameters the entry point and its
arguments.

Lastly, document the behavior and the main registers usage within the
function.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm64/head.S

@@ -306,6 +306,11 @@ primary_switched:
         /* Use a virtual address to access the UART. */
         ldr   x23, =EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Setup the arguments for start_xen and jump to C world */
+        mov   x0, x20                /* x0 := Physical offset */
+        mov   x1, x21                /* x1 := paddr(FDT) */
+        ldr   x2, =start_xen
         b     launch
 ENDPROC(real_start)
 
@@ -368,6 +373,9 @@ secondary_switched:
         /* Use a virtual address to access the UART. */
         ldr   x23, =EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Jump to C world */
+        ldr   x2, =start_secondary
         b     launch
 ENDPROC(init_secondary)
 
@@ -663,23 +671,26 @@ setup_fixmap:
         ret
 ENDPROC(setup_fixmap)
 
+/*
+ * Setup the initial stack and jump to the C world
+ *
+ * Inputs:
+ *   x0 : Argument 0 of the C function to call
+ *   x1 : Argument 1 of the C function to call
+ *   x2 : C entry point
+ *
+ * Clobbers x3
+ */
 launch:
-        PRINT("- Ready -\r\n")
+        ldr   x3, =init_data
+        add   x3, x3, #INITINFO_stack /* Find the boot-time stack */
+        ldr   x3, [x3]
+        add   x3, x3, #STACK_SIZE     /* (which grows down from the top). */
+        sub   x3, x3, #CPUINFO_sizeof /* Make room for CPU save record */
+        mov   sp, x3
 
-        ldr   x0, =init_data
-        add   x0, x0, #INITINFO_stack /* Find the boot-time stack */
-        ldr   x0, [x0]
-        add   x0, x0, #STACK_SIZE    /* (which grows down from the top). */
-        sub   x0, x0, #CPUINFO_sizeof /* Make room for CPU save record */
-        mov   sp, x0
-
-        cbnz  x22, 1f
-
-        mov   x0, x20                /* Marshal args: - phys_offset */
-        mov   x1, x21                /*               - FDT */
-        b     start_xen              /* and disappear into the land of C */
-1:
-        b     start_secondary        /* (to the appropriate entry point) */
+        /* Jump to C world */
+        br    x2
 ENDPROC(launch)
 
 /* Fail-stop */