[boot-wrapper PATCH 2/5] aarch32: simplify _switch_monitor

[Andre Przywara]

On 8/24/21 2:48 PM, Mark Rutland wrote:
> If we're lucky enough to have been booted into secure PL1, we can switch
> to monitor mode with an exception return rather than an SMC call, which
> avoids the need for boot-time vectors.
> 
> Note that while all Secure PL1 register state is accessible in Secure
> SVC mode, we must switch to Monitor mode before we set SCR.NS=1.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>

I don't know much about the subtle differences between smc, movs pc and 
eret, but from what I read this is indeed an easier way to get to MON.

Reviewed-by: Andre Przywara <andre.przywara at arm.com>

Cheers,
Andre

> ---
>   arch/aarch32/boot.S | 19 ++++---------------
>   1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/aarch32/boot.S b/arch/aarch32/boot.S
> index 82927de..e397cb0 100644
> --- a/arch/aarch32/boot.S
> +++ b/arch/aarch32/boot.S
> @@ -37,9 +37,10 @@ ASM_FUNC(_start)
>   	b	start_no_el3
>   
>   _switch_monitor:
> -	ldr	r0, =boot_vectors
> -	bl	setup_vector
> -	smc	#0
> +	adr	lr, _monitor
> +	ldr	r0, =(PSR_A | PSR_I | PSR_F | PSR_MON)
> +	msr	spsr, r0
> +	movs	pc, lr
>   
>   _monitor:
>   	/* Move the stack to Monitor mode*/
> @@ -102,18 +103,6 @@ ASM_FUNC(jump_kernel)
>   	msr	spsr_cxf, r4
>   	movs	pc, lr
>   
> -	.section .vectors
> -	.align 6
> -boot_vectors:
> -	b	.
> -	b	.
> -	b	_monitor
> -	b	.
> -	b	.
> -	b	.
> -	b	.
> -	b	.
> -
>   	.section .data
>   	.align 2
>   flag_no_el3:
>