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=-2.4 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,USER_AGENT_SANE_1 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 296E4C43603 for ; Mon, 16 Dec 2019 15:47:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0064F206E0 for ; Mon, 16 Dec 2019 15:47:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Sgdn5pob" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728480AbfLPPru (ORCPT ); Mon, 16 Dec 2019 10:47:50 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:47090 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728328AbfLPPrt (ORCPT ); Mon, 16 Dec 2019 10:47:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1576511267; 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=RgWQhpsAIADMZAP/1x3HqNJ9NVK8air/mo1dw8k/m4w=; b=Sgdn5pobUJ/ivfiBFRND0qiM5Eml3tJI4q1vuQemXgxXjnfrNGkvm3s4kCtkTFGXecGlZn aL4xTgtNibbXWwjLTHs4hVk1S1u6l5SrtW/FX8e6fYp2O2JgUzOJK48KZFnYrvIv5DQ1QK kgrP62TPXjvdSoONG/Lle+PqJRqi1Kc= Received: from mail-qv1-f70.google.com (mail-qv1-f70.google.com [209.85.219.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-436-ZfP21KXwNgaSecTEQa4hGA-1; Mon, 16 Dec 2019 10:47:44 -0500 X-MC-Unique: ZfP21KXwNgaSecTEQa4hGA-1 Received: by mail-qv1-f70.google.com with SMTP id l1so5516671qvu.13 for ; Mon, 16 Dec 2019 07:47:44 -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:user-agent; bh=RgWQhpsAIADMZAP/1x3HqNJ9NVK8air/mo1dw8k/m4w=; b=cdAhcF29TWonSVC4arTqQSq1QFKXIfMvXhKw7abyxJLRXJnTajrAxm9+Cm+hXKZtvw vI92cH2MX3jEbD9xkf2fSgfqh/PjQ5jsZFETj1eM4kFP30v0vHGJT+WBeZplwCbL1DHc kORry/3povGOSO6Dqe8Lx/sgegzmDkHXVxEH4cU/VKQfY8P7/IUfQ/VsMMqIzCdQWUEP VjX6SO9bLAY9wgQg6D7P2ndcQYkQryfdgJ9y8s/gSJyx0p4x83eR8B7/khtCZ0JoJL0k 1IR93eLkREHotClBUyVgeX8bUhxAymZK+A1/IALl1BN7c9YAGXvrStEhrwFm1+N9SpMP YXcw== X-Gm-Message-State: APjAAAUYvUGlk+rRJarMZ0Ong3cQs6nvn14tgh3vgGHbfsP0WmTtepuF 2Zf/vZoWz9wub77MwC+hDBfrG9xrDcl16x4SPTGCw/lrz6k1Uhcahdg6mPiZTbjOMrjwQbGxlY4 o7KF470BwykcsFPDTpelHTxkk X-Received: by 2002:a05:6214:108a:: with SMTP id o10mr26486295qvr.246.1576511264400; Mon, 16 Dec 2019 07:47:44 -0800 (PST) X-Google-Smtp-Source: APXvYqx1PDmhMHZHSnB6wuEZN9qkZjX10yPTV1uCaDq6nRhK2p6zGEl4P7bhy8kKpOy1w0Z1WWcikg== X-Received: by 2002:a05:6214:108a:: with SMTP id o10mr26486280qvr.246.1576511264193; Mon, 16 Dec 2019 07:47:44 -0800 (PST) Received: from xz-x1 ([104.156.64.75]) by smtp.gmail.com with ESMTPSA id b7sm6059467qkh.106.2019.12.16.07.47.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Dec 2019 07:47:42 -0800 (PST) Date: Mon, 16 Dec 2019 10:47:42 -0500 From: Peter Xu To: "Michael S. Tsirkin" 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: <20191216154742.GF83861@xz-x1> 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> <20191216103251-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191216103251-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.12.1 (2019-06-15) 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 10:33:42AM -0500, Michael S. Tsirkin wrote: > 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. In our case even an old reset_index should not matter much here imho because the worst case is we read an old reset so we stop pushing to a ring when it's just being reset and at the same time it's soft-full (so an extra user exit even race happened). But I agree it's clearer to READ_ONCE() on readers. Thanks! -- Peter Xu