From patchwork Mon Dec 12 22:37:21 2011 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paul Mackerras X-Patchwork-Id: 130922 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id E840F100A56 for ; Tue, 13 Dec 2011 09:42:15 +1100 (EST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754541Ab1LLWmO (ORCPT ); Mon, 12 Dec 2011 17:42:14 -0500 Received: from ozlabs.org ([203.10.76.45]:39962 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754472Ab1LLWls (ORCPT ); Mon, 12 Dec 2011 17:41:48 -0500 Received: by ozlabs.org (Postfix, from userid 1003) id BE50A1007E5; Tue, 13 Dec 2011 09:41:45 +1100 (EST) Date: Tue, 13 Dec 2011 09:37:21 +1100 From: Paul Mackerras To: Alexander Graf Cc: linuxppc-dev@ozlabs.org, kvm-ppc@vger.kernel.org, kvm@vger.kernel.org Subject: [PATCH v3 12/14] KVM: Add barriers to allow mmu_notifier_retry to be used locklessly Message-ID: <20111212223720.GM18868@bloggs.ozlabs.ibm.com> References: <20111212222347.GA18868@bloggs.ozlabs.ibm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20111212222347.GA18868@bloggs.ozlabs.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: kvm-ppc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm-ppc@vger.kernel.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 Acked-by: Avi Kivity --- 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 8c5c303..ec79a45 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -700,12 +700,16 @@ 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 e289486..c144132 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);