Re: [PATCH] ARM: kexec: Relax SMP validation to improve DT compatibility

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Feb 26, 2015 at 11:37:08AM +0100, Geert Uytterhoeven wrote:
> When trying to kexec into a new kernel on a platform where multiple CPU
> cores are present, but no SMP bringup code is available yet, the
> kexec_load system call fails with:
> 
>     kexec_load failed: Invalid argument
> 
> The SMP test added to machine_kexec_prepare() in commit 2103f6cba61a8b8b
> ("ARM: 7807/1: kexec: validate CPU hotplug support") wants to prohibit
> kexec on SMP platforms where it cannot disable secondary CPUs.
> However, this test is too strict: if the secondary CPUs couldn't be
> enabled in the first place, there's no need to disable them later at
> kexec time.  Hence skip the test in the absence of SMP bringup code.

Hmm.  I don't think we should relax it in this manner - I think there's
an easier solution to this.

> diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c
> index de2b085ad7535da7..8bf3b7c098881b95 100644
> --- a/arch/arm/kernel/machine_kexec.c
> +++ b/arch/arm/kernel/machine_kexec.c
> @@ -46,7 +46,8 @@ int machine_kexec_prepare(struct kimage *image)
>  	 * and implements CPU hotplug for the current HW. If not, we won't be
>  	 * able to kexec reliably, so fail the prepare operation.
>  	 */
> -	if (num_possible_cpus() > 1 && !platform_can_cpu_hotplug())
> +	if (num_possible_cpus() > 1 && platform_can_secondary_boot() &&
> +	    !platform_can_cpu_hotplug())

	if (num_online_cpus() > 1 && !platform_can_cpu_hotplug())

>  		return -EINVAL;

Neither test is actually accurate though: when we have implementations
where the secondary CPUs spin inside the kernel when they're "unplugged"
that is not sufficient to be able to kexec.

We should probably fix that, and make platform_can_cpu_hotplug() report
whether it really is possible to hotplug all secondary CPUs into such
a state that kexec can work.

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