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Date: Tue, 11 Apr 2017 14:04:30 +0200
From: Andrew Jones <drjones@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Christoffer Dall <cdall@linaro.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        James Hogan <james.hogan@imgtec.com>,
        Paul Mackerras <paulus@ozlabs.org>
Subject: Re: [PATCH 6/6] KVM: perform a wake_up in kvm_make_all_cpus_request
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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