Re: [Xen-devel] [PATCH 7/7] xen/arm32: entry: Document the purpose of r11 in the traps handler

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 19 Jan 2018, Julien Grall wrote:
> 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@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>  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 c529592d20..7f323de484 100644
> --- a/xen/arch/arm/arm32/entry.S
> +++ b/xen/arch/arm/arm32/entry.S
> @@ -136,6 +136,10 @@ 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
> @@ -246,6 +250,10 @@ DEFINE_TRAP_ENTRY_NOIRQ(fiq)
>  DEFINE_TRAP_ENTRY_NOABORT(data_abort)
>  
>  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]
> -- 
> 2.11.0
> 

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