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

[Russell King (Oracle)]

On Wed, Jun 16, 2021 at 01:10:58PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 16, 2021 at 11:34:46AM +0100, Russell King (Oracle) wrote:
> > On Wed, Jun 16, 2021 at 12:20:06PM +0200, Peter Zijlstra wrote:
> > > 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
> > 
> > If it leaves other cores with incoherent I cache, then that's already
> > a problem for SMP cores, since there could be no guarantee that the
> > modifications made by one core will be visible to some other core that
> > ends up running that code - and there is little option for userspace to
> > work around that except by pinning the thread making the modifications
> > and subsequently executing the code to a core.
> 
> That's where SYNC_CORE can help. Or you make sys_cacheflush() do a
> system wide IPI.

If it's a problem, then it needs fixing. sys_cacheflush() is used to
implement GCC's __builtin___clear_cache(). I'm not sure who added this
to gcc.

> > The same is also true of flush_icache_range() - which is used when
> > loading a kernel module. In the case Will is referring to, these alias
> > to the same code.
> 
> Yes, cache_ops_need_broadcast() seems to be missing in more places.

Likely only in places where we care about I/D coherency - as the data
cache is required to be PIPT on these SMP platforms.

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