linux-kernel - Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

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Date:   Thu, 16 Jun 2022 18:44:53 +0000
From:   oliver.upton@...ux.dev
To:     "David Laight" <David.Laight@...lab.com>
Cc:     "Raghavendra Rao Ananta" <rananta@...gle.com>,
        "Marc Zyngier" <maz@...nel.org>,
        "James Morse" <james.morse@....com>,
        "Alexandru Elisei" <alexandru.elisei@....com>,
        "Suzuki K Poulose" <suzuki.poulose@....com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>, "Peter Shier" <pshier@...gle.com>,
        "Ricardo Koller" <ricarkol@...gle.com>,
        "Oliver Upton" <oupton@...gle.com>,
        "Reiji Watanabe" <reijiw@...gle.com>,
        "Jing Zhang" <jingzhangos@...gle.com>,
        "Colton Lewis" <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        "Andrew Jones" <drjones@...hat.com>
Subject: Re: [PATCH] selftests: KVM: Handle compiler optimizations in
 ucall

June 16, 2022 11:48 AM, "David Laight" <David.Laight@...lab.com> wrote:
> No wonder I was confused.
> It's not surprising the compiler optimises it all away.
> 
> It doesn't seem right to be 'abusing' WRITE_ONCE() here.
> Just adding barrier() should be enough and much more descriptive.

I had the same thought, although I do not believe barrier() is sufficient
on its own. barrier_data() with a pointer to uc passed through
is required to keep clang from eliminating the dead store.

--
Thanks,
Oliver