[Xen-devel] [PATCH v2 25/35] xen/arm32: head: Rework and document check_cpu_mode()

[Julien Grall <julien.grall@xxxxxxx>]

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 Hypervisor Mode.

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

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    Changes in v2:
        - Patch added
---
 xen/arch/arm/arm32/head.S | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 13793e85d8..0959c4f044 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -205,6 +205,16 @@ secondary_switched:
         b     launch
 ENDPROC(init_secondary)
 
+/*
+ * Check if the CPU supports virtualization extensions and has been booted
+ * in Hypervisor mode.
+ *
+ * This function will never return when the CPU doesn't support
+ * virtualization extensions or is booted in another mode than
+ * Hypervisor mode.
+ *
+ * Clobbers r0 - r3
+ */
 check_cpu_mode:
         /* Check that this CPU has Hyp mode */
         mrc   CP32(r0, ID_PFR1)
@@ -219,15 +229,12 @@ check_cpu_mode:
         mrs   r0, cpsr
         and   r0, r0, #0x1f          /* Mode is in the low 5 bits of CPSR */
         teq   r0, #0x1a              /* Hyp Mode? */
-        beq   hyp
+        moveq pc, lr                 /* Yes, return */
 
         /* OK, we're boned. */
         PRINT("- Xen must be entered in NS Hyp mode -\r\n")
         PRINT("- Please update the bootloader -\r\n")
         b     fail
-
-hyp:    PRINT("- Xen starting in Hyp mode -\r\n")
-        mov   pc, lr
 ENDPROC(check_cpu_mode)
 
 zero_bss:
-- 
2.11.0


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