From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 0/4] KVM: Dirty logging optimization using rmap Date: Wed, 16 Nov 2011 11:06:32 +0200 Message-ID: <4EC37D18.4010609@redhat.com> References: <20111114182041.43570cdf.yoshikawa.takuya@oss.ntt.co.jp> <4EC0EC90.1090202@redhat.com> <4EC0F3D3.9090907@oss.ntt.co.jp> <4EC10BFE.7050704@redhat.com> <4EC33C0B.1060807@oss.ntt.co.jp> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: mtosatti@redhat.com, kvm@vger.kernel.org, takuya.yoshikawa@gmail.com To: Takuya Yoshikawa Return-path: Received: from mx1.redhat.com ([209.132.183.28]:36824 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754072Ab1KPJGh (ORCPT ); Wed, 16 Nov 2011 04:06:37 -0500 In-Reply-To: <4EC33C0B.1060807@oss.ntt.co.jp> Sender: kvm-owner@vger.kernel.org List-ID: On 11/16/2011 06:28 AM, Takuya Yoshikawa wrote: > (2011/11/14 21:39), Avi Kivity wrote: >> There was a patchset from Peter Zijlstra that converted mmu notifiers to >> be preemptible, with that, we can convert the mmu spinlock to a mutex, >> I'll see what happened to it. > > Interesting! > >> There is a third method of doing write protection, and that is by >> write-protecting at the higher levels of the paging hierarchy. The >> advantage there is that write protection is O(1) no matter how large the >> guest is, or the number of dirty pages. >> >> To write protect all guest memory, we just write protect the 512 PTEs at >> the very top, and leave the rest alone. When the guest writes to a >> page, we allow writes for the top-level PTE that faulted, and >> write-protect all the PTEs that it points to. > > One important point is that the guest, not GET DIRTY LOG caller, will pay > for the write protection at the timing of faults. I don't think there is a significant difference. The number of write faults does not change. The amount of work done per fault does, but not by much, thanks to the writeable bitmap. -- error compiling committee.c: too many arguments to function