From 9d534c12bf71babb76f1338029841f757191f729 Mon Sep 17 00:00:00 2001 From: Julien Grall Date: Fri, 2 Feb 2018 14:19:25 +0000 Subject: [PATCH] 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 Reviewed-by: Stefano Stabellini (cherry picked from commit dd855aa430f2da9b677c145f0c625a82aaa97110) --- xen/arch/arm/arm32/entry.S | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S index 3e48ba18d8..b66705ff50 100644 --- a/xen/arch/arm/arm32/entry.S +++ b/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] -- 2.39.5