Re: [RESEND PATCH] ARM: kexec: Fix validating CPU hotplug support

[Hu Keping]

ä 2014/11/4 18:55, Russell King - ARM Linux åé:
> On Tue, Nov 04, 2014 at 05:40:25PM +0800, HuKeping wrote:
> > Commit 2103f6cba61a8b8bea3fc1b63661d830a2125e76 added a hotplug checking in
> > machine_kexec_prepare(), but it will lead a failure when loading the
> > crash-kernel in some cases.
> >
> > Kexec utility can load the crash kernel by two ways:
> > 1. kexec -l kernel-image
> > 2. kexec -p kernel-image
> >
> > In case #1, for rapid reboot, it's correct to do the hotplug checking things,
> > for it will shut down cpus in _cpu_down() later when command "kexec -e" be
> > sent, this routine needs the support of cpu hotplug.
> >
> > In case #2, for use on panic, it's unnecessary to do the same thing, the whole
> > routing has no business with cpu shutting down. Check for cpu hot plug will
> > lead a failure to load the kernel.
>
> So what happens to the other CPUs when you kexec into the new kernel,
> possibly overwriting the instructions which those CPUs are executing?

Actually, i do think there is something wrong in the panic-rountine:
when panic comes, we clear the cpu_online_bits of other CPUs and
keep them calling cpu_relax(). That's why I post that patch ,because
we do not really shut down the CPUs.

But as your mentioned , there is another problem:
what's in the pc register of each cpu is unknown after the MMU has been
shut down.

On X86, there is a halt() before the cpu_relax(), so do you think we
need a call wfi() before cpu_relax() to keep the other CPUs on
status-WFI on ARM?

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