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

Peter Zijlstra
2021-06-16T10:21:48Z

On Wed, Jun 16, 2021 at 12:16:27PM +0200, Peter Zijlstra wrote:
> 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).

Incomplete sentence there: + then we don't need SYNC_CORE.

> 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