linux-kernel - Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus

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Message-ID: <20170411120430.7a7d6a3wzlcxntgq@kamzik.brq.redhat.com>
Date:   Tue, 11 Apr 2017 14:04:30 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <cdall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request

On Tue, Apr 11, 2017 at 01:34:49PM +0800, Paolo Bonzini wrote:
> kvm_arch_vcpu_should_kick() does cmpxchg, which already includes a
> memory barrier when it succeeds, so you need not add smp_mb() there.

When the cmpxchg() fails it only guarantees ACQUIRE semantics, meaning
the request setting may appear to happen after its completion.  This
would break our delicate vcpu->requests, vcpu->mode two-variable memory
barrier pattern that prohibits a VCPU entering guest mode with a
pending request and no IPI.  IOW, on ARM we need an explicit smp_mb()
before the cmpxchg(), otherwise it's incomplete.  I think adding a
smp_mb__before_atomic() should cover ARM and any other relaxed memory
model arches without impacting x86.

Thanks,
drew