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 751AEC43603 for ; Mon, 16 Dec 2019 15:08:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4B132206D7 for ; Mon, 16 Dec 2019 15:08:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="GNlVNF4F" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728275AbfLPPIA (ORCPT ); Mon, 16 Dec 2019 10:08:00 -0500 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:35186 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728109AbfLPPIA (ORCPT ); Mon, 16 Dec 2019 10:08:00 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1576508878; 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=AaYS2d/uqJ46OLZ/1yehXsn/hccZMKnfSM509Os6C9g=; b=GNlVNF4FOFalINF+nI9TVLLd7HnbPCkmOTsaEUHa8Qwh2Tp4R8JD01BS4YFcBaePHgjseV 8GyY34GRNvwVVrsHDmZ/Z7rkfBoK8n2FemJjn4oG2IPRqa8kYv/rI/JEIGu6Yxvb5X1k5z Eg2WgsITpBQsk2ilswsAE0QDUusl5O0= Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-325-5TvquMcFNZScuaF8TuxMcQ-1; Mon, 16 Dec 2019 10:07:56 -0500 X-MC-Unique: 5TvquMcFNZScuaF8TuxMcQ-1 Received: by mail-qt1-f197.google.com with SMTP id l25so4808872qtu.0 for ; Mon, 16 Dec 2019 07:07:55 -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=AaYS2d/uqJ46OLZ/1yehXsn/hccZMKnfSM509Os6C9g=; b=milHALK0dVuwY3t20ahRM5FCEp8LAgJnuvGyfnNlh/D4k6N/7cQpYQTu1Zmg5iQ5qk IGh/AU6fq4kaTID7DxRfeNL8Wfar30UQhPKB+7fscGEGHjn1DrcKVsg0dU6r02BK0058 KV6xOGfMED/ygCiKFqO865Z4/jqufxzL2W3s4HlQCcqswYDYFQHv6wea+b+zvFT36XMH 2pHkSP0LL62lAYhjx5lMW2VXkCkvXQWD9qC+SopRUB+liSDjA0impHGRm7lR8MrTaz6s GS/CflxdthwJYLFtwXTX6jEgORTIOE4IqVjGfIEvdt85kiv98dBMDvGX3av8/KZksxoj xriQ== X-Gm-Message-State: APjAAAXw6Hm0Gt9dKiqWgXjjRp0EVAzua6L4IILo1UGEa85ODYRHyZVU aTtAWNOmSPcJ16jqKQhHYjc0TTWZlgWyUMUrrMOg4sgJ4afEXMJU4Flc/jBn4Cq1nBVDCs7H1hi N7H1GWX4bwSUr X-Received: by 2002:ac8:787:: with SMTP id l7mr18610238qth.99.1576508875503; Mon, 16 Dec 2019 07:07:55 -0800 (PST) X-Google-Smtp-Source: APXvYqwYfnL3o3dLmvysdxBkpTp3PrdiWzj1LMSkxOlgD/ePz+FfCYUR507X8Dna+HG79+AXJS7/Tg== X-Received: by 2002:ac8:787:: with SMTP id l7mr18610206qth.99.1576508875250; Mon, 16 Dec 2019 07:07:55 -0800 (PST) Received: from xz-x1 (CPEf81d0fb19163-CMf81d0fb19160.cpe.net.fido.ca. [72.137.123.47]) by smtp.gmail.com with ESMTPSA id w1sm5343645qtk.31.2019.12.16.07.07.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Dec 2019 07:07:54 -0800 (PST) Date: Mon, 16 Dec 2019 10:07:54 -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: <20191216150754.GC83861@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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191216044619-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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, -- Peter Xu