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

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Date:   Mon, 19 Sep 2022 15:34:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     guoren@...nel.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 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 basic shape of the trap handlers looks a little like:

	if (user_mode(regs)) {
		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.