From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752904Ab3KZDC7 (ORCPT ); Mon, 25 Nov 2013 22:02:59 -0500 Received: from e23smtp06.au.ibm.com ([202.81.31.148]:38732 "EHLO e23smtp06.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752731Ab3KZDCu (ORCPT ); Mon, 25 Nov 2013 22:02:50 -0500 Message-ID: <52940F4A.4040701@linux.vnet.ibm.com> Date: Tue, 26 Nov 2013 11:02:34 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0 MIME-Version: 1.0 To: Marcelo Tosatti CC: Gleb Natapov , avi.kivity@gmail.com, "pbonzini@redhat.com Bonzini" , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Eric Dumazet , Peter Zijlstra Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc References: <1382534973-13197-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1382534973-13197-8-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <20131122191429.GA13308@amt.cnet> <65EE805B-B5DB-4BD0-A057-E5FF78D96D67@linux.vnet.ibm.com> <20131125140803.GA1489@amt.cnet> In-Reply-To: <20131125140803.GA1489@amt.cnet> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13112603-7014-0000-0000-000003F89EEE Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/25/2013 10:08 PM, Marcelo Tosatti wrote: > On Mon, Nov 25, 2013 at 02:11:31PM +0800, Xiao Guangrong wrote: >> >> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti wrote: >> >>> On Wed, Oct 23, 2013 at 09:29:25PM +0800, Xiao Guangrong wrote: >>>> It likes nulls list and we use the pte-list as the nulls which can help us to >>>> detect whether the "desc" is moved to anther rmap then we can re-walk the rmap >>>> if that happened >>>> >>>> kvm->slots_lock is held when we do lockless walking that prevents rmap >>>> is reused (free rmap need to hold that lock) so that we can not see the same >>>> nulls used on different rmaps >>>> >>>> Signed-off-by: Xiao Guangrong >>> >>> How about simplified lockless walk on the slot while rmapp entry >>> contains a single spte? (which should be the case with two-dimensional >>> paging). >>> >>> That is, grab the lock when finding a rmap with more than one spte in >>> it (and then keep it locked until the end). >> >> Hmm… that isn't straightforward and more complex than the approach >> in this patchset. Also it can drop the improvement for shadow mmu that >> gets great improvement by this patchset. > > It is not more complex, since it would remove list lockless walk. Only > the spte pointer at rmap[spte] is accessed without a lock. Its much much > simpler. > >>> For example, nothing prevents lockless walker to move into some >>> parent_ptes chain, right? >> >> No. >> >> The nulls can help us to detect this case, for parent_ptes, the nulls points >> to "shadow page" but for rmaps, the nulls points to slot.arch.rmap. There >> is no chance that the “rmap" is used as shadow page when slot-lock is held. > > The SLAB cache is the same, so entries can be reused. What prevents > a desc entry living in slot.arch.rmap to be freed and reused by a > parent_ptes desc? > We will check is_last_spte(), all the sptes on parent_ptes should be failed. And Gleb suggested to use a separate slab for rmap, that should be excellent.