From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753910AbdECO5d (ORCPT ); Wed, 3 May 2017 10:57:33 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:34025 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753283AbdECO50 (ORCPT ); Wed, 3 May 2017 10:57:26 -0400 Subject: Re: [PATCH 0/7] KVM: MMU: fast write protect To: Xiao Guangrong , mtosatti@redhat.com, avi.kivity@gmail.com, rkrcmar@redhat.com References: <20170503105224.19049-1-xiaoguangrong@tencent.com> Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, Xiao Guangrong From: Paolo Bonzini Message-ID: <878cbc47-316c-d508-a5a3-22029dee2203@redhat.com> Date: Wed, 3 May 2017 16:57:19 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/05/2017 16:50, Xiao Guangrong wrote: > Furthermore, userspace has no knowledge about if PML is enable (it > can be required from sysfs, but it is a good way in QEMU), so it is > difficult for the usespace to know when to use write-protect-all. > Maybe we can make KVM_CAP_X86_WRITE_PROTECT_ALL_MEM return false if > PML is enabled? Yes, that's a good idea. Though it's a pity that, with PML, setting the dirty bit will still do the massive walk of the rmap. At least with reset_dirty_pages it's done a little bit at a time. >> Also, I wonder how the alternative write protection mechanism would >> affect performance of the dirty page ring buffer patches. You would do >> the write protection of all memory at the end of >> kvm_vm_ioctl_reset_dirty_pages. You wouldn't even need a separate >> ioctl, which is nice. On the other hand, checkpoints would be more >> frequent and most pages would be write-protected, so it would be more >> expensive to rebuild the shadow page tables... > > Yup, write-protect-all can improve reset_dirty_pages indeed, i will > apply your idea after reset_dirty_pages is merged. > > However, we still prefer to have a separate ioctl for write-protect-all > which cooperates with KVM_GET_DIRTY_LOG to improve live migration that > should not always depend on checkpoint. Ok, I plan to merge the dirty ring pages early in 4.13 development. Paolo