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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 92E9FC43603 for ; Mon, 16 Dec 2019 15:33:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6613020717 for ; Mon, 16 Dec 2019 15:33:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="CvItLLLF" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728399AbfLPPdw (ORCPT ); Mon, 16 Dec 2019 10:33:52 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:44473 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728280AbfLPPdw (ORCPT ); Mon, 16 Dec 2019 10:33:52 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1576510430; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Z0ykzv4O9XDflANBBucUp6ssbent/uqQftfOjZxhGN0=; b=CvItLLLFf9H0OcxsysiMIYLsVR993UsqrVKMkYsTASsyRTNbjoURpshFMP1kK9iGXKnGaQ mg5uX2nXfGMYeE3HPJlDj7wxZ9s3bQy4jMzWVKalFf4vzitTXsxrQ/SHuBWBh4HWbtdnWz Aq0+rKZ8agxql3pc30/68RGVyfBb0AU= Received: from mail-qt1-f198.google.com (mail-qt1-f198.google.com [209.85.160.198]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-280-R_s1NlYMOwem18pCbqPcqw-1; Mon, 16 Dec 2019 10:33:49 -0500 X-MC-Unique: R_s1NlYMOwem18pCbqPcqw-1 Received: by mail-qt1-f198.google.com with SMTP id z12so244584qts.15 for ; Mon, 16 Dec 2019 07:33:49 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Z0ykzv4O9XDflANBBucUp6ssbent/uqQftfOjZxhGN0=; b=E1po9uamIFPsMIznCtJUr77hB8GHVnnXrBu6CiB/EkD8p6nHSPZPxNjOK7MCSOSskZ Ohlmka4gcChP3pumNdUKDNCEjDighUwh9lf9TpM4kuRAD00tjhTkuUSB1L0WHtVwaxa7 DWhfbG85XntBaYT91gJM3za13j+yLprdsnSr/xvRX85jzCBOUiGyNKlkQKSSM5elZ7Cj /W6Jv2Z645ATLT87Rt849XILNJNs5HEuX7iX4xvtd26p0VWCbFyIpERAC7F2y2L10txc yNNR2LNAvvN/l8Hu+rUOh8Z6fGmzasfJT3X6ICeNCgA4FZgZCROIAzYEJh6dusR+/X0D aVMQ== X-Gm-Message-State: APjAAAUfMx+9MDiAhelw0tUxkceECk1LqfenB7JYUTMn+hzCkMKleSuu WrO+qSuS5/dZLADKQXVRxN2u7VLwYcW5C7SjoC0wRtNyNTOnYFxjUuG3/DVgf+xq1RQ1BMw7ZZ4 l+Z25quXF7yFDajys3jm6HHm3 X-Received: by 2002:a05:620a:b18:: with SMTP id t24mr27387986qkg.341.1576510428730; Mon, 16 Dec 2019 07:33:48 -0800 (PST) X-Google-Smtp-Source: APXvYqwYwsAN4Q+QC29+/ynoBvhgn+sVgs19/Aa51xUTnXtyYxuSUmHPy54H02pcDC57nW5rMIQk2w== X-Received: by 2002:a05:620a:b18:: with SMTP id t24mr27387965qkg.341.1576510428508; Mon, 16 Dec 2019 07:33:48 -0800 (PST) Received: from redhat.com ([212.199.108.238]) by smtp.gmail.com with ESMTPSA id b191sm6118550qkg.43.2019.12.16.07.33.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Dec 2019 07:33:47 -0800 (PST) Date: Mon, 16 Dec 2019 10:33:42 -0500 From: "Michael S. Tsirkin" To: Peter Xu Cc: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Sean Christopherson , "Dr . David Alan Gilbert" , Vitaly Kuznetsov Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking Message-ID: <20191216103251-mutt-send-email-mst@kernel.org> References: <20191129213505.18472-1-peterx@redhat.com> <20191129213505.18472-5-peterx@redhat.com> <20191211063830-mutt-send-email-mst@kernel.org> <20191211205952.GA5091@xz-x1> <20191211172713-mutt-send-email-mst@kernel.org> <46ceb88c-0ddd-0d9a-7128-3aa5a7d9d233@redhat.com> <20191215173302.GB83861@xz-x1> <20191216044619-mutt-send-email-mst@kernel.org> <20191216150754.GC83861@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191216150754.GC83861@xz-x1> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 16, 2019 at 10:07:54AM -0500, Peter Xu wrote: > On Mon, Dec 16, 2019 at 04:47:36AM -0500, Michael S. Tsirkin wrote: > > On Sun, Dec 15, 2019 at 12:33:02PM -0500, Peter Xu wrote: > > > On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote: > > > > >>> What depends on what here? Looks suspicious ... > > > > >> > > > > >> Hmm, I think maybe it can be removed because the entry pointer > > > > >> reference below should be an ordering constraint already? > > > > > > > > entry->xxx depends on ring->reset_index. > > > > > > Yes that's true, but... > > > > > > entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)]; > > > /* barrier? */ > > > next_slot = READ_ONCE(entry->slot); > > > next_offset = READ_ONCE(entry->offset); > > > > > > ... I think entry->xxx depends on entry first, then entry depends on > > > reset_index. So it seems fine because all things have a dependency? > > > > Is reset_index changed from another thread then? > > If yes then you want to read reset_index with READ_ONCE. > > That includes a dependency barrier. > > There're a few readers, but only this function will change it > (kvm_dirty_ring_reset). Thanks, Then you don't need any barriers in this function. readers need at least READ_ONCE. > -- > Peter Xu