Re: [PATCH v4 1/2] arm64: entry: Skip single stepping into interrupt handlers

[Sumit Garg]

 On Fri, 16 Dec 2022 at 22:14, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Thu, Dec 15, 2022 at 07:59:02PM +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>
>
> FWIW, this looks reasonable to me; I have a couple of minor style/structure
> comments below.

Thanks Mark for your review.

>
> > ---
> >  arch/arm64/kernel/entry-common.c | 18 +++++++++++++++++-
> >  1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> > index cce1167199e3..53bcb1902f2f 100644
> > --- a/arch/arm64/kernel/entry-common.c
> > +++ b/arch/arm64/kernel/entry-common.c
> > @@ -471,19 +471,35 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
> >       do_interrupt_handler(regs, handler);
> >       irq_exit_rcu();
> >
> > -     arm64_preempt_schedule_irq();
> > +     /* Don't reschedule in case we are single stepping */
> > +     if (!(regs->pstate & DBG_SPSR_SS))
> > +             arm64_preempt_schedule_irq();
>
> Please change arm64_preempt_schedule_irq() to take the regs as an argument, and
> put this test inside arm64_preempt_schedule_irq(). That way all the
> decision-making about whether to preempt is in one place.
>
> That can go immediately after the need_irq_preemption() test.

Okay, I will change arm64_preempt_schedule_irq() accordingly.

>
> >
> >       exit_to_kernel_mode(regs);
> >  }
> > +
> >  static void noinstr el1_interrupt(struct pt_regs *regs,
> >                                 void (*handler)(struct pt_regs *))
> >  {
> > +     unsigned long reg;
>
> Please s/reg/mdscr/. That way it's harder to confuse with 'regs', it's clearer
> that it's the MDSCR_ELx value, and if we have to save/restore any other
> registers in future it'll be obvious how to name things.
>

Ack.

-Sumit

> Thanks,
> Mark.
>
> > +
> > +     /* Disable single stepping within interrupt handler */
> > +     if (regs->pstate & DBG_SPSR_SS) {
> > +             reg = read_sysreg(mdscr_el1);
> > +             write_sysreg(reg & ~DBG_MDSCR_SS, mdscr_el1);
> > +     }
> > +
> >       write_sysreg(DAIF_PROCCTX_NOIRQ, daif);
> >
> >       if (IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI) && !interrupts_enabled(regs))
> >               __el1_pnmi(regs, handler);
> >       else
> >               __el1_irq(regs, handler);
> > +
> > +     if (regs->pstate & DBG_SPSR_SS) {
> > +             write_sysreg(DAIF_PROCCTX_NOIRQ | PSR_D_BIT, daif);
> > +             write_sysreg(reg, mdscr_el1);
> > +     }
> >  }
> >
> >  asmlinkage void noinstr el1h_64_irq_handler(struct pt_regs *regs)
> > --
> > 2.34.1
> >