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

A branch in the success case can be avoided by inverting the branch
condition. At the same time, remove a pointless comment as Xen can only
run at EL2.

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>

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index df797a1573e4880d89019bcf75fca0ee1b8550d6..fbcc792adeb78710cd18d472b0440388b0b1f711 100644 (file)
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -350,6 +350,13 @@ secondary_switched:
         b     launch
 ENDPROC(init_secondary)
 
+/*
+ * Check if the CPU has been booted in Hypervisor mode.
+ * This function will never return when the CPU is booted in another mode
+ * than Hypervisor mode.
+ *
+ * Clobbers x0 - x5
+ */
 check_cpu_mode:
         PRINT("- Current EL ")
         mrs   x5, CurrentEL
@@ -359,15 +366,13 @@ check_cpu_mode:
         /* Are we in EL2 */
         cmp   x5, #PSR_MODE_EL2t
         ccmp  x5, #PSR_MODE_EL2h, #0x4, ne
-        b.eq  el2 /* Yes */
-
+        b.ne  1f /* No */
+        ret
+1:
         /* OK, we're boned. */
         PRINT("- Xen must be entered in NS EL2 mode -\r\n")
         PRINT("- Please update the bootloader -\r\n")
         b fail
-
-el2:    PRINT("- Xen starting at EL2 -\r\n")
-        ret
 ENDPROC(check_cpu_mode)
 
 zero_bss: