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From:   Guo Ren <guoren@kernel.org>
Date:   Mon, 17 Jul 2023 07:33:25 +0800
Message-ID: <CAJF2gTTt23iSDG_m4ihPhXhYDrz3Xnih=KGLx_ayBLbzPqaTaQ@mail.gmail.com>
Subject: Re: [PATCH] riscv: entry: Fixup do_trap_break from kernel side
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     arnd@arndb.de, palmer@rivosinc.com, tglx@linutronix.de,
        luto@kernel.org, conor.dooley@microchip.com, heiko@sntech.de,
        jszhang@kernel.org, lazyparser@gmail.com, falcon@tinylab.org,
        chenhuacai@kernel.org, apatel@ventanamicro.com,
        atishp@atishpatra.org, mark.rutland@arm.com, bjorn@kernel.org,
        palmer@dabbelt.com, bjorn@rivosinc.com, daniel.thompson@linaro.org,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org, stable@vger.kernel.org,
        Guo Ren <guoren@linux.alibaba.com>
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On Mon, Jul 10, 2023 at 4:02=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Sun, Jul 09, 2023 at 10:30:22AM +0800, Guo Ren wrote:
> > On Wed, Jul 5, 2023 at 12:40=E2=80=AFAM Peter Zijlstra <peterz@infradea=
d.org> wrote:
> > >
> > > On Sat, Jul 01, 2023 at 10:57:07PM -0400, guoren@kernel.org wrote:
> > > > From: Guo Ren <guoren@linux.alibaba.com>
> > > >
> > > > The irqentry_nmi_enter/exit would force the current context into in=
_interrupt.
> > > > That would trigger the kernel to dead panic, but the kdb still need=
s "ebreak" to
> > > > debug the kernel.
> > > >
> > > > Move irqentry_nmi_enter/exit to exception_enter/exit could correct =
handle_break
> > > > of the kernel side.
> > >
> > > This doesn't explain much if anything :/
> > >
> > > I'm confused (probably because I don't know RISC-V very well), what's
> > > EBREAK and how does it happen?
> > EBREAK is just an instruction of riscv which would rise breakpoint exce=
ption.
> >
> >
> > >
> > > Specifically, if EBREAK can happen inside an local_irq_disable() regi=
on,
> > > then the below change is actively wrong. Any exception/interrupt that
> > > can happen while local_irq_disable() must be treated like an NMI.
> > When the ebreak happend out of local_irq_disable region, but
> > __nmi_enter forces handle_break() into in_interupt() state. So how
>
> And why is that a problem? I think I'm missing something fundamental
> here...
The irqentry_nmi_enter() would force the current context to get
in_interrupt=3Dtrue, although ebreak happens in the context which is
in_interrupt=3Dfalse.
A lot of checking codes, such as:
        if (in_interrupt())
                panic("Fatal exception in interrupt");
It would make the kernel panic, but we don't panic; we want back to the she=
ll.
eg:
echo BUG > /sys/kernel/debug/provoke-crash/DIRECT

>
> > about:
> >
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index f910dfccbf5d..69f7043a98b9 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/irq.h>
> >  #include <linux/kexec.h>
> >  #include <linux/entry-common.h>
> > +#include <linux/context_tracking.h>
> >
> >  #include <asm/asm-prototypes.h>
> >  #include <asm/bug.h>
> > @@ -285,12 +286,18 @@ asmlinkage __visible __trap_section void
> > do_trap_break(struct pt_regs *regs)
> >                 handle_break(regs);
> >
> >                 irqentry_exit_to_user_mode(regs);
> > -       } else {
> > +       } else if (in_interrupt()){
> >                 irqentry_state_t state =3D irqentry_nmi_enter(regs);
> >
> >                 handle_break(regs);
> >
> >                 irqentry_nmi_exit(regs, state);
> > +       } else {
> > +               enum ctx_state prev_state =3D exception_enter();
> > +
> > +               handle_break(regs);
> > +
> > +               exception_exit(prev_state);
> >         }
> >  }
>
> That's wrong. If you want to make it conditional, you have to look at
> !(regs->status & SR_IE) (that's the interrupt enable flag of the
> interrupted context, right?)
>
> When you hit an EBREAK when IRQs were disabled, you must be NMI like.
>
> But making it conditional like this makes it really hard to write a
> handler though, it basically must assume it will be NMI contetx (because
> it can't know) so there is no point in sometimes not doing NMI context.


--=20
Best Regards
 Guo Ren