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Date:   Wed, 1 Jul 2020 11:24:28 +0100
From:   Will Deacon <will@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Kees Cook <keescook@chromium.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jason Wang <jasowang@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Josh Triplett <josh@joshtriplett.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        linux-arm-kernel@lists.infradead.org,
        Sami Tolvanen <samitolvanen@google.com>,
        linux-alpha@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Matt Turner <mattst88@gmail.com>,
        virtualization@lists.linux-foundation.org,
        Android Kernel Team <kernel-team@android.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when
 CLANG_LTO=y
Message-ID: <20200701102427.GD14959@willie-the-truck>
References: <20200630173734.14057-1-will@kernel.org>
 <20200630173734.14057-19-will@kernel.org>
 <CANpmjNPOO=AVsVJMdL8sq03jwHsDR_1_FfWccwaLKEBRn1RFtA@mail.gmail.com>
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On Tue, Jun 30, 2020 at 09:47:30PM +0200, Marco Elver wrote:
> On Tue, 30 Jun 2020 at 19:39, Will Deacon <will@kernel.org> wrote:
> >
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> >
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> >
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
> >  arch/arm64/kernel/vdso/Makefile   |  2 +-
> >  arch/arm64/kernel/vdso32/Makefile |  2 +-
> >  3 files changed, 65 insertions(+), 2 deletions(-)
> >  create mode 100644 arch/arm64/include/asm/rwonce.h
> 
> This seems reasonable, given we can't realistically tell the compiler
> about dependent loads. What (if any), is the performance impact? I
> guess this also heavily depends on the actual silicon.

Right, it depends both on the CPU micro-architecture and also the workload.
When we ran some basic tests, the overhead wasn't greater than the benefit
seen by enabling LTO, so it seems like a reasonable trade-off (given that
LTO is a dependency for CFI, so it's not just about performance).

Will