From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751902Ab3KYG3S (ORCPT ); Mon, 25 Nov 2013 01:29:18 -0500 Received: from e28smtp06.in.ibm.com ([122.248.162.6]:60226 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750958Ab3KYG3O (ORCPT ); Mon, 25 Nov 2013 01:29:14 -0500 Message-ID: <5292EE2F.5090305@linux.vnet.ibm.com> Date: Mon, 25 Nov 2013 14:29:03 +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: Xiao Guangrong , 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> In-Reply-To: <65EE805B-B5DB-4BD0-A057-E5FF78D96D67@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13112506-9574-0000-0000-00000ABD0557 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/25/2013 02:11 PM, 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. > >> >> 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. > >> >> Also, there is no guarantee of termination (as long as sptes are >> deleted with the correct timing). BTW, can't see any guarantee of >> termination for rculist nulls either (a writer can race with a lockless >> reader indefinately, restarting the lockless walk every time). > > Hmm, that can be avoided by checking dirty-bitmap before rewalk, > that means, if the dirty-bitmap has been set during lockless write-protection, > it�s unnecessary to write-protect its sptes. Your idea? This idea is based on the fact that the number of rmap is limited by RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap, we can break the rewalk at once, in the case of deleting, we can only rewalk RMAP_RECYCLE_THRESHOLD times.