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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E19C5C433F5 for ; Sat, 8 Oct 2022 17:36:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229892AbiJHRgX (ORCPT ); Sat, 8 Oct 2022 13:36:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55292 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230404AbiJHRgD (ORCPT ); Sat, 8 Oct 2022 13:36:03 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A34A041526; Sat, 8 Oct 2022 10:35:53 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 5FF7960AE5; Sat, 8 Oct 2022 17:35:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E23CC433D7; Sat, 8 Oct 2022 17:35:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1665250553; bh=nAFecTafpi6Cui7vA6b8YkSuKqU56pFcMIrM2l6VfXQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BJ79rTDZDoZOoqyJ1p4euj7MzBAC4RxT7l1N9Iy0VIoZjXdoV8YF2DYVUomZrPgZJ UusXvel6i15sqSnYL9payXgqHy9nCDYdhs2SLj/peFcHVwsGAdd4vQYt04Fms8MefY n0golxj0X0OydUL1DHB7B5ncQNksTTImYnmYjABdGXeyFXvwxEMGpfVZQP4fRjgZ8i HYtTZAcmfbQFMOam3HjFBPVl+pgxt71ZlFeuP6bCyAVOXTLIlXHlr+wNjFquTOZiRX rRqj1YugIgbl2xgs4xtDtFK8WOkGXluEJO8Pn613Vs5w/RYaEtv/WzTWeLVOynb0hT V4MVh/JzSrXBg== Date: Sat, 8 Oct 2022 20:35:47 +0300 From: Jarkko Sakkinen To: Sean Christopherson Cc: Chao Peng , 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: 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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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. BR, Jarkko