From mboxrd@z Thu Jan  1 00:00:00 1970
From: Josh Poimboeuf <jpoimboe@kernel.org>
Subject: Re: [PATCH v2 19/24] xtensa/cpu: Make sure cpu_die() doesn't return
Date: Tue, 14 Feb 2023 12:19:09 -0800
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To: Max Filippov <jcmvbkbc@gmail.com>
Cc: juri.lelli@redhat.com, dalias@libc.org, linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, peterz@infradead.org, catalin.marinas@arm.com, dave.hansen@linux.intel.com, x86@kernel.org, jiaxun.yang@flygoat.com, linux-mips@vger.kernel.org, bsegall@google.com, guoren@kernel.org, hpa@zytor.com, sparclinux@vger.kernel.org, kernel@xen0n.name, will@kernel.org, vschneid@redhat.com, f.fainelli@gmail.com, vincent.guittot@linaro.org, ysato@users.sourceforge.jp, chenhuacai@kernel.org, linux@armlinux.org.uk, linux-csky@vger.kernel.org, mingo@redhat.com, bcm-kernel-feedback-list@broadcom.com, mgorman@suse.de, mattst88@gmail.com, linux-xtensa@linux-xtensa.org, paulmck@kernel.org, richard.henderson@linaro.org, npiggin@gmail.com, ink@jurassic.park.msu.ru, rostedt@goodmis.org, loongarch@lis

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@kernel.org> wro=
te:
> > On Tue, Feb 14, 2023 at 08:55:32AM +0100, Philippe Mathieu-Daud=C3=A9 w=
rote:
> > > 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.
>=20
> 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.

--=20
Josh