xen/arm32: entry: Document the purpose of r11 in the traps handler

It took me a bit of time to understand why __DEFINE_TRAP_ENTRY is
storing the original stack pointer in r11. It is working in pair with
return_traps_entry where sp will be restored from r11.

This is fine because per the AAPCS r11 must be preserved by the
subroutine. So in return_from_trap, r11 will still contain the original
stack pointer.

Add some documentation in the code to point the 2 sides to each other.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit dd855aa430)

xen/arch/arm/arm32/entry.S

@@ -127,6 +127,10 @@ trap_##trap:                                                            \
         cpsie iflags;                                                   \
         adr lr, return_from_trap;                                       \
         mov r0, sp;                                                     \
+        /*                                                              \
+         * Save the stack pointer in r11. It will be restored after the \
+         * trap has been handled (see return_from_trap).                \
+         */                                                             \
         mov r11, sp;                                                    \
         bic sp, #7; /* Align the stack pointer (noop on guest trap) */  \
         b do_trap_##trap
@@ -217,6 +221,10 @@ DEFINE_TRAP_ENTRY_NOIRQ(irq)
 DEFINE_TRAP_ENTRY_NOIRQ(fiq)
 
 return_from_trap:
+        /*
+         * Restore the stack pointer from r11. It was saved on exception
+         * entry (see __DEFINE_TRAP_ENTRY).
+         */
         mov sp, r11
 ENTRY(return_to_new_vcpu32)
         ldr r11, [sp, #UREGS_cpsr]