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Date:   Wed, 16 Jun 2021 12:16:27 +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
Message-ID: <YMnPezLs6vb418Wz@hirez.programming.kicks-ass.net>
References: <cover.1623813516.git.luto@kernel.org>
 <2142129092ff9aa00e600c42a26c4015b7f5ceec.1623813516.git.luto@kernel.org>
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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