linux-kernel - Re: [PATCH V5 07/11] riscv: convert to generic entry

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Message-ID: <CAJF2gTRdkmemEWsDYhVXb8KD0PS6b1VAPu_MfeZ+Rmf2qEGa6Q@mail.gmail.com>
Date:   Tue, 20 Sep 2022 14:36:33 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     arnd@...db.de, palmer@...osinc.com, tglx@...utronix.de,
        luto@...nel.org, conor.dooley@...rochip.com, heiko@...ech.de,
        jszhang@...nel.org, lazyparser@...il.com, falcon@...ylab.org,
        chenhuacai@...nel.org, apatel@...tanamicro.com,
        atishp@...shpatra.org, palmer@...belt.com,
        paul.walmsley@...ive.com, mark.rutland@....com,
        zouyipeng@...wei.com, bigeasy@...utronix.de,
        David.Laight@...lab.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V5 07/11] riscv: convert to generic entry

On Mon, Sep 19, 2022 at 9:34 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Sun, Sep 18, 2022 at 11:52:42AM -0400, guoren@...nel.org wrote:
>
> > @@ -123,18 +126,22 @@ int handle_misaligned_store(struct pt_regs *regs);
> >
> >  asmlinkage void __trap_section do_trap_load_misaligned(struct pt_regs *regs)
> >  {
> > +     irqentry_state_t state = irqentry_enter(regs);
> >       if (!handle_misaligned_load(regs))
> >               return;
> >       do_trap_error(regs, SIGBUS, BUS_ADRALN, regs->epc,
> >                     "Oops - load address misaligned");
> > +     irqentry_exit(regs, state);
> >  }
> >
> >  asmlinkage void __trap_section do_trap_store_misaligned(struct pt_regs *regs)
> >  {
> > +     irqentry_state_t state = irqentry_enter(regs);
> >       if (!handle_misaligned_store(regs))
> >               return;
> >       do_trap_error(regs, SIGBUS, BUS_ADRALN, regs->epc,
> >                     "Oops - store (or AMO) address misaligned");
> > +     irqentry_exit(regs, state);
> >  }
> >  #endif
> >  DO_ERROR_INFO(do_trap_store_fault,
> > @@ -158,6 +165,8 @@ static inline unsigned long get_break_insn_length(unsigned long pc)
> >
> >  asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
> >  {
> > +     irqentry_state_t state = irqentry_enter(regs);
> > +
> >  #ifdef CONFIG_KPROBES
> >       if (kprobe_single_step_handler(regs))
> >               return;
>
> FWIW; on x86 I've classified many of the 'from-kernel' traps as
> NMI-like, since those traps can happen from any context, including with
> IRQs disabled.
The do_trap_break is for ebreak instruction, not NMI. RISC-V NMI has
separate CSR. ref:

This proposal adds support for resumable non-maskable interrupts
(RNMIs) to RISC-V. The extension adds four new CSRs (`mnepc`,
`mncause`, `mnstatus`, and `mnscratch`) to hold the interrupted state,
and a new instruction to resume from the RNMI handler.

>
> The basic shape of the trap handlers looks a little like:
>
>         if (user_mode(regs)) {
If nmi comes from user_mode, why we using
irqenrty_enter/exit_from/to_user_mode instead of
irqentry_nmi_enter/exit?

>                 irqenrty_enter_from_user_mode(regs);
>                 do_user_trap();
>                 irqentry_exit_to_user_mode(regs);
>         } else {
>                 irqentry_state_t state = irqentry_nmi_enter(regs);
>                 do_kernel_trap();
>                 irqentry_nmi_exit(regs, state);
>         }
>
> Not saying you have to match Risc-V in this patch-set, just something to
> consider.
I think the shape of the riscv NMI handler looks a little like this:

asmlinkage __visible __trap_section void do_trap_nmi(struct pt_regs *regs)
{
                 irqentry_state_t state = irqentry_nmi_enter(regs);
                 do_nmi_trap();
                 irqentry_nmi_exit(regs, state);
}

-- 
Best Regards
 Guo Ren