From patchwork Tue Dec 6 06:13:13 2011 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paul Mackerras X-Patchwork-Id: 129558 Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from ozlabs.org (localhost [IPv6:::1]) by ozlabs.org (Postfix) with ESMTP id ED497101987 for ; Tue, 6 Dec 2011 17:22:29 +1100 (EST) Received: by ozlabs.org (Postfix) id 72E5F1007DE; Tue, 6 Dec 2011 17:21:25 +1100 (EST) Delivered-To: linuxppc-dev@ozlabs.org Received: by ozlabs.org (Postfix, from userid 1003) id 7182F1007DD; Tue, 6 Dec 2011 17:21:25 +1100 (EST) Date: Tue, 6 Dec 2011 17:13:13 +1100 From: Paul Mackerras To: Alexander Graf Subject: [PATCH 11/13] KVM: Add barriers to allow mmu_notifier_retry to be used locklessly Message-ID: <20111206061313.GO12389@drongo> References: <20111206060156.GD12389@drongo> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20111206060156.GD12389@drongo> User-Agent: Mutt/1.5.21 (2010-09-15) Cc: linuxppc-dev@ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+patchwork-incoming=ozlabs.org@lists.ozlabs.org Sender: linuxppc-dev-bounces+patchwork-incoming=ozlabs.org@lists.ozlabs.org This adds an smp_wmb in kvm_mmu_notifier_invalidate_range_end() and an smp_rmb in mmu_notifier_retry() so that mmu_notifier_retry() will give the correct answer when called without kvm->mmu_lock being held. PowerPC Book3S HV KVM wants to use a bitlock per guest page rather than a single global spinlock in order to improve the scalability of updates to the guest MMU hashed page table, and so needs this. Signed-off-by: Paul Mackerras --- include/linux/kvm_host.h | 14 +++++++++----- virt/kvm/kvm_main.c | 6 +++--- 2 files changed, 12 insertions(+), 8 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index c6a2ec9..17319ad 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -672,12 +672,16 @@ static inline int mmu_notifier_retry(struct kvm_vcpu *vcpu, unsigned long mmu_se if (unlikely(vcpu->kvm->mmu_notifier_count)) return 1; /* - * Both reads happen under the mmu_lock and both values are - * modified under mmu_lock, so there's no need of smb_rmb() - * here in between, otherwise mmu_notifier_count should be - * read before mmu_notifier_seq, see - * mmu_notifier_invalidate_range_end write side. + * Ensure the read of mmu_notifier_count happens before the read + * of mmu_notifier_seq. This interacts with the smp_wmb() in + * mmu_notifier_invalidate_range_end to make sure that the caller + * either sees the old (non-zero) value of mmu_notifier_count or + * the new (incremented) value of mmu_notifier_seq. + * PowerPC Book3s HV KVM calls this under a per-page lock + * rather than under kvm->mmu_lock, for scalability, so + * can't rely on kvm->mmu_lock to keep things ordered. */ + smp_rmb(); if (vcpu->kvm->mmu_notifier_seq != mmu_seq) return 1; return 0; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index d9cfb78..ad2a912 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -357,11 +357,11 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, * been freed. */ kvm->mmu_notifier_seq++; + smp_wmb(); /* * The above sequence increase must be visible before the - * below count decrease but both values are read by the kvm - * page fault under mmu_lock spinlock so we don't need to add - * a smb_wmb() here in between the two. + * below count decrease, which is ensured by the smp_wmb above + * in conjunction with the smp_rmb in mmu_notifier_retry(). */ kvm->mmu_notifier_count--; spin_unlock(&kvm->mmu_lock);