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 96422C43603 for ; Wed, 4 Dec 2019 19:52:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 67C122073B for ; Wed, 4 Dec 2019 19:52:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZsP+JtW+" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728053AbfLDTwl (ORCPT ); Wed, 4 Dec 2019 14:52:41 -0500 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:23745 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727867AbfLDTwh (ORCPT ); Wed, 4 Dec 2019 14:52:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575489157; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=2HusBtOWadZiLTPK1SmC8n0gdc/q9WlraTHbywKYh/s=; b=ZsP+JtW+SPCx3zoVpwPZr9WbEolxp98kw0mGaMTZ2Ho8TLxSVdwNwWJ3g9t5ZCHZZmqkm6 fRVSfwT/DKbe3LMuAEkar+cAAb0l2g4wfXVaouJaIoMf5QSxj3/fpJdtFFSFayCx+XjIPY OzA1yjEWx9pTF9JO1QUQ3fl3Dd4c7d8= Received: from mail-qt1-f200.google.com (mail-qt1-f200.google.com [209.85.160.200]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-305-Wk397ZESNdWq_c_XloFGYQ-1; Wed, 04 Dec 2019 14:52:33 -0500 Received: by mail-qt1-f200.google.com with SMTP id x8so728036qtq.14 for ; Wed, 04 Dec 2019 11:52:33 -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:content-transfer-encoding :in-reply-to:user-agent; bh=COps3gzSWJZrk5fQQE220Gmmy+Anmpkd5GgCFKaWMT0=; b=BAsV9PlJF2+wTiCruIBG2aL/yjgLPFgPiAghvhOV3JHSVCSso7l+CHxskkh+GhNKE/ b/l3HOdqDcoZezR2DJ4MhVefjl0OzPS6y984lFGWKI8h2JKrdjTMiagIvqXKkUTOUckq 4dzh9x6VqTd9z4Vhg7VSAnNssT1fmdppKOlDaFKXn3Jbd81R+PDTiLnVxGRuR75jS9yg WmntrpmIUdXORZ+RmJ5+OSsEHhygV3ZaUvCPWKJp60cSWCCpHnn8oTe/s8EpKD0rTalN xO+XF+SNL0X9XNpnS4M5PLg9xYdvFzF8gfGoJtB68F/FjUsjTrFbaRa3O7KL1QWngiYA swfw== X-Gm-Message-State: APjAAAXu+64ubFtR6sbDlsZZfIpBlM/GZZtHr4s2BF7GVSJrYuPRly91 3ElrhdZo6rtos2X+aZSlKObhdhl+sKRhpU8uz0s7lwh5QSbbxBpI9IGjVMxV2pOTy7TFt/5pUEE QHGj0E9jZ83Ll X-Received: by 2002:a37:9345:: with SMTP id v66mr4783226qkd.195.1575489152870; Wed, 04 Dec 2019 11:52:32 -0800 (PST) X-Google-Smtp-Source: APXvYqwmm8jnKWcmw4+PPJFHhDL8CRLCxfukJjz5uZsgYx4DOnQ3LuvLF7qvQ1/vOwQrDYghrXaqbA== X-Received: by 2002:a37:9345:: with SMTP id v66mr4783190qkd.195.1575489152545; Wed, 04 Dec 2019 11:52:32 -0800 (PST) Received: from xz-x1 ([104.156.64.74]) by smtp.gmail.com with ESMTPSA id t9sm453529qkt.112.2019.12.04.11.52.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Dec 2019 11:52:31 -0800 (PST) Date: Wed, 4 Dec 2019 14:52:30 -0500 From: Peter Xu To: Paolo Bonzini Cc: Jason Wang , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Sean Christopherson , "Dr . David Alan Gilbert" , Vitaly Kuznetsov , "Michael S. Tsirkin" Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking Message-ID: <20191204195230.GF19939@xz-x1> References: <20191129213505.18472-1-peterx@redhat.com> <20191129213505.18472-5-peterx@redhat.com> <1355422f-ab62-9dc3-2b48-71a6e221786b@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.11.4 (2019-03-13) X-MC-Unique: Wk397ZESNdWq_c_XloFGYQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote: > On 04/12/19 11:38, Jason Wang wrote: > >> > >> +=C2=A0=C2=A0=C2=A0 entry =3D &ring->dirty_gfns[ring->dirty_index & (r= ing->size - 1)]; > >> +=C2=A0=C2=A0=C2=A0 entry->slot =3D slot; > >> +=C2=A0=C2=A0=C2=A0 entry->offset =3D offset; > >=20 > >=20 > > Haven't gone through the whole series, sorry if it was a silly question > > but I wonder things like this will suffer from similar issue on > > virtually tagged archs as mentioned in [1]. >=20 > There is no new infrastructure to track the dirty pages---it's just a > different way to pass them to userspace. >=20 > > Is this better to allocate the ring from userspace and set to KVM > > instead? Then we can use copy_to/from_user() friends (a little bit slow > > on recent CPUs). >=20 > Yeah, I don't think that would be better than mmap. Yeah I agree, because I didn't see how copy_to/from_user() helped to do icache/dcache flushings... Some context here: Jason raised this question offlist first on whether we should also need these flush_dcache_cache() helpers for operations like kvm dirty ring accesses. I feel like it should, however I've got two other questions, on: - if we need to do flush_dcache_page() on kernel modified pages (assuming the same page has mapped to userspace), then why don't we need flush_cache_page() too on the page, where flush_cache_page() is defined not-a-nop on those archs? - assuming an arch has not-a-nop impl for flush_[d]cache_page(), would atomic operations like cmpxchg really work for them (assuming that ISAs like cmpxchg should depend on cache consistency). Sorry I think these are for sure a bit out of topic for kvm dirty ring patchset, but since we're at it, I'm raising the questions up in case there're answers.. Thanks, --=20 Peter Xu