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

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Date:   Fri, 17 Jun 2022 09:28:00 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'oliver.upton@...ux.dev'" <oliver.upton@...ux.dev>,
        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" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

On Thu, Jun 16, 2022 at 09:54:16PM +0000, David Laight wrote:
> From: oliver.upton@...ux.dev
> > Sent: 16 June 2022 19:45
> 
> > 
> > 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.
> 
> A barrier() (full memory clobber) ought to be stronger than
> the partial one than barrier_data() generates.
> 
> I can't quite decide whether you need a barrier() both sides
> of the 'magic write'.
> Plausibly the compiler could discard the on-stack data
> after the barrier() and before the 'magic write'.
> 
> Certainly putting the 'magic write' inside a asm block
> that has a memory clobber is a more correct solution.

Indeed, since the magic write is actually a guest MMIO write, then
it should be using writeq().

Thanks,
drew