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

[Stephen Warren <swarren@wwwdotorg.org>]

On 02/26/2015 10:42 AM, Russell King - ARM Linux wrote:
> 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())

I can't remember the call stack here. Is num_online_cpus() guaranteed 
not to change from this point through to when the kexec actually happens?

>
>>   		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.
>