Re: [PATCH v5 1/2] arm64: entry: Skip single stepping into interrupt handlers

[Will Deacon]

On Mon, Dec 19, 2022 at 03:54:51PM +0530, Sumit Garg wrote:
> Currently on systems where the timer interrupt (or any other
> fast-at-human-scale periodic interrupt) is active then it is impossible
> to step any code with interrupts unlocked because we will always end up
> stepping into the timer interrupt instead of stepping the user code.
> 
> The common user's goal while single stepping is that when they step then
> the system will stop at PC+4 or PC+I for a branch that gets taken
> relative to the instruction they are stepping. So, fix broken single step
> implementation via skipping single stepping into interrupt handlers.
> 
> The methodology is when we receive an interrupt from EL1, check if we
> are single stepping (pstate.SS). If yes then we save MDSCR_EL1.SS and
> clear the register bit if it was set. Then unmask only D and leave I set.
> On return from the interrupt, set D and restore MDSCR_EL1.SS. Along with
> this skip reschedule if we were stepping.
> 
> Suggested-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/entry-common.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index cce1167199e3..688d1ef8e864 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -231,11 +231,15 @@ DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
>  #define need_irq_preemption()	(IS_ENABLED(CONFIG_PREEMPTION))
>  #endif
>  
> -static void __sched arm64_preempt_schedule_irq(void)
> +static void __sched arm64_preempt_schedule_irq(struct pt_regs *regs)
>  {
>  	if (!need_irq_preemption())
>  		return;
>  
> +	/* Don't reschedule in case we are single stepping */
> +	if (!(regs->pstate & DBG_SPSR_SS))
> +		return;

Hmm, isn't this the common case? PSTATE.SS will usually be clear, no?

>  	 * Note: thread_info::preempt_count includes both thread_info::count
>  	 * and thread_info::need_resched, and is not equivalent to
> @@ -471,19 +475,33 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
>  	do_interrupt_handler(regs, handler);
>  	irq_exit_rcu();
>  
> -	arm64_preempt_schedule_irq();
> +	arm64_preempt_schedule_irq(regs);
>  
>  	exit_to_kernel_mode(regs);
>  }
> +
>  static void noinstr el1_interrupt(struct pt_regs *regs,
>  				  void (*handler)(struct pt_regs *))
>  {
> +	unsigned long mdscr;
> +
> +	/* Disable single stepping within interrupt handler */
> +	if (regs->pstate & DBG_SPSR_SS) {
> +		mdscr = read_sysreg(mdscr_el1);
> +		write_sysreg(mdscr & ~DBG_MDSCR_SS, mdscr_el1);
> +	}

I think this will break the implicit handling of kernel {break,watch}points.

Sadly, I think any attempts to workaround the issues here are likely just
to push the problems around. We really need to overhaul the debug exception
handling logic we have, which means I need to get back to writing up a
proposal.

Will