[PATCH] arm64: emulate aarch32 CP15 barriers if needed

[Ming Lei]

On Sun, Nov 10, 2013 at 7:25 PM, Catalin Marinas
<catalin.marinas at arm.com> wrote:
> On 10 Nov 2013, at 09:23, Ming Lei <ming.lei at canonical.com> wrote:
>> CP15BEN of SCTLR_EL1 may not be implemented, so we need
>> to emulate these three CP15 barriers to avoid breaking
>> aarch32 applications since they can be used in user mode.
>>
>> Cc: Catalin Marinas <catalin.marinas at arm.com>
>> Cc: Will Deacon <will.deacon at arm.com>
>> Signed-off-by: Ming Lei <ming.lei at canonical.com>
>
> Can you describe which aarch32 applications is this breaking?  These
> barriers have been deprecated in ARMv7 and removed in ARMv8 (same as the
> SWP instruction).  User space is supposed to use the kuser helpers which
> provide the right barriers for the architecture.  Compiler intrinsics
> for ARMv7 already generate (inline) the new ARMv7 barriers.

I know one armv7 business software(closed source) uses cp15 dmb,
and actually any armv7 user applications may use these instructions
since they are allowed in user mode.

>
> So for compat support, I consider this similar to other instructions we
> will not emulate like SWP and certain unaligned accesses.  IOW, arm64
> compat only supports non-deprecated ARMv7 features.

That will inevitably break some current armv7 user space, and cause
compatibility problem since aarch64 claims to support armv7 user
application.

Also looks CP15BEN of SCTLR_EL1 is for such purpose.


Thanks,
--
Ming Lei