linux-kernel - Re: [PATCH v2 19/24] xtensa/cpu: Make sure cpu

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Message-ID: <20230214201909.flqc3g3njnbl5hun@treble>
Date:   Tue, 14 Feb 2023 12:19:09 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Max Filippov <jcmvbkbc@...il.com>
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Subject: Re: [PATCH v2 19/24] xtensa/cpu: Make sure cpu_die() doesn't return

On Tue, Feb 14, 2023 at 11:48:41AM -0800, Max Filippov wrote:
> On Tue, Feb 14, 2023 at 10:23 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > On Tue, Feb 14, 2023 at 08:55:32AM +0100, Philippe Mathieu-Daudé wrote:
> > > Can you update the documentation along? Currently we have:
> > >
> > >   /*
> > >    * Called from the idle thread for the CPU which has been shutdown.
> > >    *
> > >    * Note that we disable IRQs here, but do not re-enable them
> > >    * before returning to the caller. This is also the behaviour
> > >    * of the other hotplug-cpu capable cores, so presumably coming
> > >    * out of idle fixes this.
> > >    */
> >
> > void __ref cpu_die(void)
> > {
> >         idle_task_exit();
> >         local_irq_disable();
> >         __asm__ __volatile__(
> >                         "       movi    a2, cpu_restart\n"
> >                         "       jx      a2\n");
> >
> >         BUG();
> > }
> >
> > Hm, not only is the comment wrong, but it seems to be branching to
> > cpu_restart?  That doesn't seem right at all.
> 
> Perhaps the name is a bit misleading. The CPU that enters 'cpu_restart'
> loops there until a call to 'boot_secondary' releases it, after which it goes
> to '_startup'. So it is a restart, but not immediate.

Ah, I see.  That sounds similar to what Xen does.

-- 
Josh