[PATCH v4] arm64: run-time detection for aarch32 support

[Suzuki.Poulose@arm.com (Suzuki K. Poulose)]

On 18/12/15 17:46, Mark Rutland wrote:
> On Fri, Dec 18, 2015 at 05:03:11PM +0000, Suzuki K. Poulose wrote:
>> On 18/12/15 16:00, Yury Norov wrote:
>>> Kernel option COMPAT defines the ability of executing aarch32 binaries.
>> As per ARM ARM, AArch32 only ID register values are unknown if AArch32 is
>> not implemented. So I think we need to skip accessing the AArch32 ID registers
>> everywhere (feature tracking), if the CPU doesn't supports it, to avoid
>> unnecessary SANITY failures and TAINTing the kernel.
>
> That all sounds good to me.
>
> After boot-time we should also fail hotplug of a CPU that doesn't
> support AArch32, if we decided at boot-time that AArch32 was supported
> accross the system. That should probably be added to your early cpu
> feature verification [1].

You are right. I think we could add this as system capability to arm64_feautres
which would make the check automatic. Also, the compat_elf_arch() check won't
have to always to read_system_reg(), something like ARM64_HAS_32BIT_EL0.

Thanks
Suzuki