Can cores have different CPU features in a SMP system?

[Jun Sun]

Hi, Suzuki,

I was looking at kernel 3.18 and drew the earlier conclusion.

I'm now looking at kernel v4.7-rc4.  It appears the logic is

  - cpu 0 sets up elf_hwcap (init_cpu_ftr_reg(), init_cpu_features() )
  - other cpus call verify_local_cpu_capabilities() in
secondary_start_kernel() and parks itself if it finds any missing
features.

Is this what you see? It seems to be slightly different from your description.

BTW, this is exactly the kind of enforcement I was hoping to see.  I
couldn't be happier now. :) Thanks for your pointer.

Jun

On Tue, Aug 2, 2016 at 3:09 AM, Suzuki K Poulose <Suzuki.Poulose at arm.com> wrote:
> On 01/08/16 20:08, Jun Sun wrote:
>>
>> I looked at the kernel source code again.  Kernel currently only
>> detects CPU features of CPU0 and use it for all other CPUs.   In other
>> words, it is assuming all cores must have the same CPU features, which
>> in a sense is a form of "enforcing".
>
>
> On arm64 kernels, the CPU features are detected based on their availability
> on all the CPUs. Hence, for e.g, if one cluster doesn't have crypto, kernel
> doesn't expose the ELF AUX_HWCAP  crypto bits to the application (which
> should be
> consulted by the application before doing a CPU Feature specific operation).
>
>>
>> As a result, it is definitely no-no to have cores with different CPU
>> features, which is what I suspected.  Case closed. :)
>
>
> It is not recommended to have cores with different CPU features, but as
> mentioned above, arm64 kernel can cope up with the situation and disallow
> any features that aren't available across all the CPUs.
> See arch/arm64/kernel/cpufeature.c for more information.
>
> Cheers
> Suzuki
>
>
>
>>
>> Jun
>>
>> On Fri, Jul 29, 2016 at 2:30 PM, Jun Sun <jsun at junsun.net> wrote:
>>>
>>> One specific example is that if a phone has two clusters.  Cores in
>>> one cluster has v8 crypto extension and cores in the other don't.
>>> Will this work?
>>>
>>> My gut feeling is no, since many apps query cpu features at run-time.
>>> And process can migrate among cores.  If an app starts on a core with
>>> CE and migrates to another core without CE.  Illegal instruction fault
>>> will happen.
>>>
>>> Just wanna confirm if this is indeed the case.  Surprisingly google
>>> search did not yield any meaningful answers.
>>>
>>> Cheers.
>>>
>>> Jun
>>
>>
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>