From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C03A9C432C0 for ; Mon, 2 Dec 2019 22:45:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8AFB820722 for ; Mon, 2 Dec 2019 22:45:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725941AbfLBWpD (ORCPT ); Mon, 2 Dec 2019 17:45:03 -0500 Received: from mga06.intel.com ([134.134.136.31]:9178 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725853AbfLBWpD (ORCPT ); Mon, 2 Dec 2019 17:45:03 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 02 Dec 2019 14:45:02 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,270,1571727600"; d="scan'208";a="208249916" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by fmsmga008.fm.intel.com with ESMTP; 02 Dec 2019 14:45:02 -0800 Date: Mon, 2 Dec 2019 14:45:02 -0800 From: Sean Christopherson To: Ben Gardon Cc: kvm@vger.kernel.org, Paolo Bonzini , Peter Feiner , Peter Shier , Junaid Shahid , Jim Mattson Subject: Re: [RFC PATCH 06/28] kvm: mmu: Replace mmu_lock with a read/write lock Message-ID: <20191202224501.GF8120@linux.intel.com> References: <20190926231824.149014-1-bgardon@google.com> <20190926231824.149014-7-bgardon@google.com> <20191127184736.GF22227@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191127184736.GF22227@linux.intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Nov 27, 2019 at 10:47:36AM -0800, Sean Christopherson wrote: > On Thu, Sep 26, 2019 at 04:18:02PM -0700, Ben Gardon wrote: > > Replace the KVM MMU spinlock with a read/write lock so that some parts of > > the MMU can be made more concurrent in future commits by switching some > > write mode aquisitions to read mode. A read/write lock was chosen over > > other synchronization options beause it has minimal initial impact: this > > change simply changes all uses of the MMU spin lock to an MMU read/write > > lock, in write mode. This change has no effect on the logic of the code > > and only a small performance penalty. > > > > Other, more invasive options were considered for synchronizing access to > > the paging structures. Sharding the MMU lock to protect 2MB chunks of > > addresses, as the main MM does, would also work, however it makes > > acquiring locks for operations on large regions of memory expensive. > > Further, the parallel page fault handling algorithm introduced later in > > this series does not require exclusive access to the region of memory > > for which it is handling a fault. > > > > There are several disadvantages to the read/write lock approach: > > 1. The reader/writer terminology does not apply well to MMU operations. > > 2. Many operations require exclusive access to a region of memory > > (often a memslot), but not all of memory. The read/write lock does not > > facilitate this. > > 3. Contention between readers and writers can still create problems in > > the face of long running MMU operations. > > > > Despite these issues,the use of a read/write lock facilitates > > substantial improvements over the monolithic locking scheme. > > > > Signed-off-by: Ben Gardon > > --- > > arch/x86/kvm/mmu.c | 106 +++++++++++++++++++------------------ > > arch/x86/kvm/page_track.c | 8 +-- > > arch/x86/kvm/paging_tmpl.h | 8 +-- > > arch/x86/kvm/x86.c | 4 +- > > include/linux/kvm_host.h | 3 +- > > virt/kvm/kvm_main.c | 34 ++++++------ > > 6 files changed, 83 insertions(+), 80 deletions(-) > > > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > > index 56587655aecb9..0311d18d9a995 100644 > > --- a/arch/x86/kvm/mmu.c > > +++ b/arch/x86/kvm/mmu.c > > @@ -2446,9 +2446,9 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu, > > flush |= kvm_sync_page(vcpu, sp, &invalid_list); > > mmu_pages_clear_parents(&parents); > > } > > - if (need_resched() || spin_needbreak(&vcpu->kvm->mmu_lock)) { > > I gather there is no equivalent to spin_needbreak() for r/w locks? Is it > something that can be added? Losing spinlock contention detection will > negatively impact other flows, e.g. fast zapping all pages will no longer > drop the lock to allow insertion of SPTEs into the new generation of MMU. Just saw that fast zap is explicitly noted in the cover letter. Is there anything beyond a spin_needbreak() implementation that's needed to support fast zap? > > + if (need_resched()) { > > kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush); > > - cond_resched_lock(&vcpu->kvm->mmu_lock); > > + cond_resched_rwlock_write(&vcpu->kvm->mmu_lock); > > flush = false; > > } > > }