Can cores have different CPU features in a SMP system?

[Suzuki K Poulose]

On 03/08/16 10:00, Suzuki K Poulose wrote:
> On 02/08/16 18:23, Jun Sun wrote:
>> 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() )
>
> On arm64 CPU 0 doesn't set up elf_hwcap. It initialises the CPU feature register
> infrastructure which keeps track of the system wide safe values for the ID
> registers. init_cpu_features() initialises the table with the values from
> the boot CPU. This will also be used to detect if any secondary CPU has
> variations in features which could affect the normal functioning of Linux (via
> update_cpu_features() -> check_update_ftr_reg() ).
>
> The elf_hwcap and the system CPU_HWCAP bits are initialised only after all
> the boot time active CPUs are online, from
> setup_cpu_features()-> { setup_elf_hwcaps(), setup_feature_capabilities() }

And the values are set based on the system wide safe values from the CPU feature
infrastructure (using read_system_reg()).

Suzuki