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

[Sumit Garg]

Hi Will,

Thanks for your review.

On Thu, 26 Jan 2023 at 19:09, Will Deacon <will@xxxxxxxxxx> wrote:
>
> 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?
>

Ah I see, looks like a copy paste error from v4. This check should be instead:

     /* Don't reschedule in case we are single stepping */
     if (regs->pstate & DBG_SPSR_SS)
             return;

Thanks for catching this, I will correct it in the next version.

> >        * 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.
>

Can you please elaborate here? AFAICS, this change will only omit the
interrupt handler while stepping.

> 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.
>

I will be very happy to assist you if you can help me understand the
problem here.

BTW, patch #2 should be an independent fix from patch #1. Can you pull
that alone?

-Sumit

> Will