[PATCH] arm64: emulate aarch32 CP15 barriers if needed

[Catalin Marinas]

On 10 Nov 2013, at 12:08, Ming Lei <ming.lei at canonical.com> wrote:
> 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,

OK, so that’s even easier ;)

Next they’ll complain about the performance of such operations (imagine
some fast user-space locking in a database software trapping for each
barrier).

> and actually any armv7 user applications may use these instructions
> since they are allowed in user mode.

There are others allowed like SWP but we decided not to emulate them
since they’ve been deprecated for a long time.

>> 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.

It supports armv7 user applications using *non-deprecated& features
(and EABI).

> Also looks CP15BEN of SCTLR_EL1 is for such purpose.

If it’s just a matter of enabling it in SCTLR_EL1, that would be easier
but the hardware may simply not implement these instructions in ARMv8
(I guess that’s why you want emulation). My position is that such legacy
software should be upgraded to make use of the kuser helpers (if ARMv6
and earlier support is required) or just DMB/DSB/ISB if optimised for
ARMv7.

Catalin