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Date:   Wed, 16 Jun 2021 12:20:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     x86@kernel.org, Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Russell King <linux@armlinux.org.uk>,
        linux-arm-kernel@lists.infradead.org, Will Deacon <will@kernel.org>
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
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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