linux-kernel - Re: [PATCH v3 01/22] hardirq/nmi: Allow nested nmi

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Message-ID: <20200219103126.33f67cf3@gandalf.local.home>
Date:   Wed, 19 Feb 2020 10:31:26 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v3 01/22] hardirq/nmi: Allow nested nmi_enter()

On Wed, 19 Feb 2020 15:47:25 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> Since there are already a number of sites (ARM64, PowerPC) that
> effectively nest nmi_enter(), lets make the primitive support this
> before adding even more.
> 


>  void SMIException(struct pt_regs *regs)
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -71,7 +71,7 @@ extern void irq_exit(void);
>  		printk_nmi_enter();				\
>  		lockdep_off();					\
>  		ftrace_nmi_enter();				\
> -		BUG_ON(in_nmi());				\
> +		BUG_ON(in_nmi() == NMI_MASK);			\
>  		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
>  		rcu_nmi_enter();				\
>  		trace_hardirq_enter();				\
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -26,13 +26,13 @@
>   *         PREEMPT_MASK:	0x000000ff
>   *         SOFTIRQ_MASK:	0x0000ff00
>   *         HARDIRQ_MASK:	0x000f0000
> - *             NMI_MASK:	0x00100000
> + *             NMI_MASK:	0x00f00000
>   * PREEMPT_NEED_RESCHED:	0x80000000
>   */
>  #define PREEMPT_BITS	8
>  #define SOFTIRQ_BITS	8
>  #define HARDIRQ_BITS	4
> -#define NMI_BITS	1
> +#define NMI_BITS	4
>  

Probably should document somewhere (in a comment above nmi_enter()?)
that we allow nmi_enter() to nest up to 15 times.

-- Steve