Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE

[Peter Zijlstra]

On Tue, Jun 15, 2021 at 08:21:12PM -0700, Andy Lutomirski wrote:
> On arm32, the only way to safely flush icache from usermode is to call
> cacheflush(2).  This also handles any required pipeline flushes, so
> membarrier's SYNC_CORE feature is useless on arm.  Remove it.

So SYNC_CORE is there to help an architecture that needs to do something
per CPU. If I$ invalidation is broadcast and I$ invalidation also
triggers the flush of any uarch caches derived from it (if there are
any).

Now arm_syscall() NR(cacheflush) seems to do flush_icache_user_range(),
which, if I read things right, end up in arch/arm/mm/*.S, but that
doesn't consider cache_ops_need_broadcast().

Will suggests that perhaps ARM 11MPCore might need this due to their I$
flush maybe not being broadcast