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 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8B6CCC433F5 for ; Mon, 10 Oct 2022 08:43:31 +0000 (UTC) Received: from localhost ([::1]:45712 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ohoNm-0004NR-Ff for qemu-devel@archiver.kernel.org; Mon, 10 Oct 2022 04:43:30 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56758) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ohoAi-0000e7-AZ for qemu-devel@nongnu.org; Mon, 10 Oct 2022 04:30:01 -0400 Received: from mga12.intel.com ([192.55.52.136]:62254) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ohoAc-0002Ff-Rv for qemu-devel@nongnu.org; Mon, 10 Oct 2022 04:30:00 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1665390594; x=1696926594; h=date:from:to:cc:subject:message-id:reply-to:references: mime-version:in-reply-to; bh=5AUxEww8V8qWvvG5bCArmS7RBRQUNOLUMKgyDwR1O0k=; b=BYSQ0/oCN/EMFDWfwu/4Js8Mx1DMpkSDcbvWuR8/h9ItBNN8GzIDuNmb +NXvERrhsJxsuiJnJDDt7zWo+SgpwQWic5YzvyergRHwOh3A0AZ53myTV scBzpa1uHwAzLePPBdW9u0BnswUHkXJqDZJBj3El6AGFc07Zy+QB4FxI/ nlENjdaMelw87RXp0KcR6Ks7VnGgEKWToiPtU6K/CxESXwbd4ZPpDUOtY j/zX3DXM5Ljbe4LFynejElXnk2qOY0b9bL7sLZHmc0GcCP8gD35GsVHUQ RPMqWVPajvFym8R6iZ2cROU00B39+wgU+xUV2UJg3EHUBIeiTa6cd6iMe g==; X-IronPort-AV: E=McAfee;i="6500,9779,10495"; a="283899554" X-IronPort-AV: E=Sophos;i="5.95,173,1661842800"; d="scan'208";a="283899554" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Oct 2022 01:29:51 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6500,9779,10495"; a="656853060" X-IronPort-AV: E=Sophos;i="5.95,173,1661842800"; d="scan'208";a="656853060" Received: from chaop.bj.intel.com (HELO localhost) ([10.240.193.75]) by orsmga008.jf.intel.com with ESMTP; 10 Oct 2022 01:29:40 -0700 Date: Mon, 10 Oct 2022 16:25:07 +0800 From: Chao Peng To: Jarkko Sakkinen Cc: Sean Christopherson , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Shuah Khan , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com, Muchun Song , wei.w.wang@intel.com Subject: Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based private memory Message-ID: <20221010082507.GA3144879@chaop.bj.intel.com> References: <20220915142913.2213336-1-chao.p.peng@linux.intel.com> <20220915142913.2213336-3-chao.p.peng@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: none client-ip=192.55.52.136; envelope-from=chao.p.peng@linux.intel.com; helo=mga12.intel.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Chao Peng Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Sat, Oct 08, 2022 at 08:35:47PM +0300, Jarkko Sakkinen wrote: > On Sat, Oct 08, 2022 at 07:15:17PM +0300, Jarkko Sakkinen wrote: > > On Sat, Oct 08, 2022 at 12:54:32AM +0300, Jarkko Sakkinen wrote: > > > On Fri, Oct 07, 2022 at 02:58:54PM +0000, Sean Christopherson wrote: > > > > On Fri, Oct 07, 2022, Jarkko Sakkinen wrote: > > > > > On Thu, Oct 06, 2022 at 03:34:58PM +0000, Sean Christopherson wrote: > > > > > > On Thu, Oct 06, 2022, Jarkko Sakkinen wrote: > > > > > > > On Thu, Oct 06, 2022 at 05:58:03PM +0300, Jarkko Sakkinen wrote: > > > > > > > > On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote: > > > > > > > > > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two > > > > > > > > > additional KVM memslot fields private_fd/private_offset to allow > > > > > > > > > userspace to specify that guest private memory provided from the > > > > > > > > > private_fd and guest_phys_addr mapped at the private_offset of the > > > > > > > > > private_fd, spanning a range of memory_size. > > > > > > > > > > > > > > > > > > The extended memslot can still have the userspace_addr(hva). When use, a > > > > > > > > > single memslot can maintain both private memory through private > > > > > > > > > fd(private_fd/private_offset) and shared memory through > > > > > > > > > hva(userspace_addr). Whether the private or shared part is visible to > > > > > > > > > guest is maintained by other KVM code. > > > > > > > > > > > > > > > > What is anyway the appeal of private_offset field, instead of having just > > > > > > > > 1:1 association between regions and files, i.e. one memfd per region? > > > > > > > > > > > > Modifying memslots is slow, both in KVM and in QEMU (not sure about Google's VMM). > > > > > > E.g. if a vCPU converts a single page, it will be forced to wait until all other > > > > > > vCPUs drop SRCU, which can have severe latency spikes, e.g. if KVM is faulting in > > > > > > memory. KVM's memslot updates also hold a mutex for the entire duration of the > > > > > > update, i.e. conversions on different vCPUs would be fully serialized, exacerbating > > > > > > the SRCU problem. > > > > > > > > > > > > KVM also has historical baggage where it "needs" to zap _all_ SPTEs when any > > > > > > memslot is deleted. > > > > > > > > > > > > Taking both a private_fd and a shared userspace address allows userspace to convert > > > > > > between private and shared without having to manipulate memslots. > > > > > > > > > > Right, this was really good explanation, thank you. > > > > > > > > > > Still wondering could this possibly work (or not): > > > > > > > > > > 1. Union userspace_addr and private_fd. > > > > > > > > No, because userspace needs to be able to provide both userspace_addr (shared > > > > memory) and private_fd (private memory) for a single memslot. > > > > > > Got it, thanks for clearing my misunderstandings on this topic, and it > > > is quite obviously visible in 5/8 and 7/8. I.e. if I got it right, > > > memblock can be partially private, and you dig the shared holes with > > > KVM_MEMORY_ENCRYPT_UNREG_REGION. We have (in Enarx) ATM have memblock > > > per host mmap, I was looking into this dilated by that mindset but makes > > > definitely sense to support that. > > > > For me the most useful reference with this feature is kvm_set_phys_mem() > > implementation in privmem-v8 branch. Took while to find it because I did > > not have much experience with QEMU code base. I'd even recommend to mention > > that function in the cover letter because it is really good reference on > > how this feature is supposed to be used. That's a good point, I can mention that if people find useful. > > While learning QEMU code, I also noticed bunch of comparison like this: > > if (slot->flags | KVM_MEM_PRIVATE) > > I guess those could be just replaced with unconditional fills as it does > not do any harm, if KVM_MEM_PRIVATE is not set. Make sense, thanks. Chao > > BR, Jarkko