[PATCH 2/4] arm64: entry: clarify entry/exit helpers

[Joey Gouly]

On Mon, Aug 02, 2021 at 03:07:31PM +0100, Mark Rutland wrote:
> When entering an exception, we must perform irq/context state management
> before we can use instrumentable C code. Similary, when exiting an

typo: Similarily

> exception we cannot use instrumentable C code after we perform
> irq/context state management.
> 
> Originally, we'd intended that the enter_from_*() and exit_to_*()
> helpers would enforce this by virtue of being the first and last
> functions called, respectively, in an exception handler. However, as
> they now call instrumentable code themselves, this is not as clearly
> true.
> 
> To make this more robust, this patch splits the irq/context state
> management into separate helpers, with all the helpers commented to make
> their intended purpose more obvious.
> 
> In exit_to_kernel_mode() we'll now check TFSR_EL1 before we assert that
> IRQs are disabled, but this ordering is not important, and other than
> this there should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Joey Gouly <joey.gouly at arm.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/kernel/entry-common.c | 70 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 63 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 6f7a98d8d60f..6dc64f99f185 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -26,10 +26,14 @@
>  #include <asm/system_misc.h>
>  
>  /*
> + * Handle IRQ/context state management when entering from kernel mode.
> + * Before this function is called it is not safe to call regular kernel code,
> + * intrumentable code, or any code which may trigger an exception.
> + *
>   * This is intended to match the logic in irqentry_enter(), handling the kernel
>   * mode transitions only.
>   */
> -static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
> +static __always_inline void __enter_from_kernel_mode(struct pt_regs *regs)
>  {
>  	regs->exit_rcu = false;
>  
> @@ -45,20 +49,26 @@ static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
>  	lockdep_hardirqs_off(CALLER_ADDR0);
>  	rcu_irq_enter_check_tick();
>  	trace_hardirqs_off_finish();
> +}
>  
> +static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
> +{
> +	__enter_from_kernel_mode(regs);
>  	mte_check_tfsr_entry();
>  }
>  
>  /*
> + * Handle irq/context state management when exiting to kernel mode.
Uppercase IRQ to match the other comments.
> + * after this function returns it is not safe to call regular kernel code,
'After' to match the other comments.
> + * intrumentable code, or any code which may trigger an exception.
> + *
>   * This is intended to match the logic in irqentry_exit(), handling the kernel
>   * mode transitions only, and with preemption handled elsewhere.
>   */
> -static void noinstr exit_to_kernel_mode(struct pt_regs *regs)
> +static __always_inline void __exit_to_kernel_mode(struct pt_regs *regs)
>  {
>  	lockdep_assert_irqs_disabled();
>  
> -	mte_check_tfsr_exit();
> -
>  	if (interrupts_enabled(regs)) {
>  		if (regs->exit_rcu) {
>  			trace_hardirqs_on_prepare();
> @@ -75,7 +85,18 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs)

Reviewed-by: Joey Gouly <joey.gouly at arm.com>