From: David Laight <David.Laight@ACULAB.COM>
To: "'oliver.upton@linux.dev'" <oliver.upton@linux.dev>
Cc: Raghavendra Rao Ananta <rananta@google.com>,
Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
Alexandru Elisei <alexandru.elisei@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Peter Shier <pshier@google.com>,
"Ricardo Koller" <ricarkol@google.com>,
Oliver Upton <oupton@google.com>,
"Reiji Watanabe" <reijiw@google.com>,
Jing Zhang <jingzhangos@google.com>,
"Colton Lewis" <coltonlewis@google.com>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kselftest@vger.kernel.org"
<linux-kselftest@vger.kernel.org>,
"Andrew Jones" <drjones@redhat.com>
Subject: RE: [PATCH] selftests: KVM: Handle compiler optimizations in ucall
Date: Thu, 16 Jun 2022 21:54:16 +0000 [thread overview]
Message-ID: <2ec9ecbfb13d422ab6cda355ff011c9f@AcuMS.aculab.com> (raw)
In-Reply-To: <7b1040c48bc9b2986798322c336660ab@linux.dev>
From: oliver.upton@linux.dev
> Sent: 16 June 2022 19:45
>
> June 16, 2022 11:48 AM, "David Laight" <David.Laight@aculab.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.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
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next prev parent reply other threads:[~2022-06-16 21:55 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-15 18:57 [PATCH] selftests: KVM: Handle compiler optimizations in ucall Raghavendra Rao Ananta
2022-06-16 12:02 ` Andrew Jones
2022-06-16 15:58 ` David Laight
2022-06-16 16:25 ` Andrew Jones
2022-06-16 16:48 ` David Laight
2022-06-16 18:44 ` oliver.upton
2022-06-16 21:54 ` David Laight [this message]
2022-06-17 7:28 ` Andrew Jones
2022-06-17 9:22 ` Paolo Bonzini
2022-06-16 12:24 ` Paolo Bonzini
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