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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 DB639C0015E for ; Thu, 27 Jul 2023 17:13:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:From:Subject:Message-ID: References:Mime-Version:In-Reply-To:Date:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=HRVct3AuRXnXkqQTmExkXMvMCpmF1Yu2FDhgwXnm9y4=; b=zltbtfkhf6bxuugbPhhV3uYcQ5 uvIyNjJcUddvplq74GsME5720G9by2bz1S2U6o0JZt39e6u4s4KZ/SMf0Emjo8cItXwQGmdscn05x 92REf/QIHxB+FOLXFk9d9oNY3lK9S1hBXjHgykV+gbytmAUaGCNBHUfGBIxfz48RlTyAorELeQ8Ba Bgz2d4uDM37lrsdWpBWlqeh6oaFvW81PaIzv4SjO+6E4i2m0WS+afI2ORDNw9zYuX8BolifxY0eDz Gy1RCsqWMdgP0om7gXQO9iSqtbIy1nzEG5GyPl2syArAcYeDzf+zUHeB7v8+WsyMuuW3BOq6Ng7bI irmlpJHA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qP4Y9-00HAKj-1j; Thu, 27 Jul 2023 17:13:17 +0000 Received: from mail-pl1-x649.google.com ([2607:f8b0:4864:20::649]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1qP4Y3-00HAE7-1R for linux-riscv@lists.infradead.org; Thu, 27 Jul 2023 17:13:15 +0000 Received: by mail-pl1-x649.google.com with SMTP id d9443c01a7336-1bbb34b091dso8228795ad.0 for ; Thu, 27 Jul 2023 10:13:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1690477989; x=1691082789; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=F7oDMpfc0bW6NSyfuHNNNYlN2HFPFEuCSHvjU51+NBs=; b=uR617wLpg6qNB677nCTpj/mRcHCn7jeMQ6T975dwFWafb05LBOA5GXBHggNBovfIzX O/7M392ccdTOGZqesE1qC7szLmXDUVbpTkdUSAnDKesUZGTHf6YrHw7iRdcLemcslWSD KrRYixDmKkhke5BhBNDcGGoexHbctICnVSugeNNjkFTNcd3g+7vTx3m4xPX98nUwGt3/ mWhf+uFvINzhg7lFN3O3GR8jmx04AReHHh1Ek1sP09ZwG+oxGwlWe7n39MiREf5a36wi QECqs1dJt2n4+euTMFmAQH0Oiz0RKLy4HgS6MVL8RVa78gxf6JRosPxL/mvaakJouVeF ho3w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1690477989; x=1691082789; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=F7oDMpfc0bW6NSyfuHNNNYlN2HFPFEuCSHvjU51+NBs=; b=bA7px1pckdt3LaaFaIEpKiH8/NMDEtEyv++b8jk+GaaLiDtqajMaWZut+2UQ3aJplD eovehCls3P+wino8U2kCz8dxiiw6D+AyrtSdBT6SqSTmGnuRp7HRtlVM3ktb5OGfMbI8 B0KsHR/+oL1hLFGTDtVZQKHFnYEhLQfMp4PWO6C+0sr88LAd7OAklAktvCstOVKyFDtU M945UiNvWQXwXGcqWf4by4tFSVATOo7G2zdT5uDNk2mc7exnm3HvnWX2GkzeYTTkytH9 w//oG/Prp1T585TZQr/fnQsWZl2VJJFv0lp1aIUollR9LRlWDVBChtZPrIy3Sp8zKHOl fV4g== X-Gm-Message-State: ABy/qLbu2FoGlQKQ7Kkgj7DuDVJWdMrIT1V5Dtcx9TMrYVoVQYy4hXnU Lyek/ldqYE64qTccLUzv0KdsgpLLlkg= X-Google-Smtp-Source: APBJJlEiCX9DyxliShqr2xPxTiyej7fUi+PkS+4LO77eNeghg1Fiv4b0fvD5YH2w9mP2/cy4q9w9vNXHRRI= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:903:22c6:b0:1b5:2b14:5f2c with SMTP id y6-20020a17090322c600b001b52b145f2cmr24803plg.4.1690477989357; Thu, 27 Jul 2023 10:13:09 -0700 (PDT) Date: Thu, 27 Jul 2023 10:13:07 -0700 In-Reply-To: Mime-Version: 1.0 References: <20230718234512.1690985-1-seanjc@google.com> <20230718234512.1690985-13-seanjc@google.com> Message-ID: Subject: Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory From: Sean Christopherson To: Fuad Tabba Cc: Paolo Bonzini , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , "Matthew Wilcox (Oracle)" , Andrew Morton , Paul Moore , James Morris , "Serge E. Hallyn" , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, Chao Peng , Jarkko Sakkinen , Yu Zhang , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , Vlastimil Babka , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata , "Kirill A . Shutemov" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230727_101311_519010_9DCE968B X-CRM114-Status: GOOD ( 25.93 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Thu, Jul 27, 2023, Fuad Tabba wrote: > Hi Sean, > > > ... > > > @@ -5134,6 +5167,16 @@ static long kvm_vm_ioctl(struct file *filp, > > case KVM_GET_STATS_FD: > > r = kvm_vm_ioctl_get_stats_fd(kvm); > > break; > > + case KVM_CREATE_GUEST_MEMFD: { > > + struct kvm_create_guest_memfd guest_memfd; > > + > > + r = -EFAULT; > > + if (copy_from_user(&guest_memfd, argp, sizeof(guest_memfd))) > > + goto out; > > + > > + r = kvm_gmem_create(kvm, &guest_memfd); > > + break; > > + } > > I'm thinking line of sight here, by having this as a vm ioctl (rather > than a system iocl), would it complicate making it possible in the > future to share/donate memory between VMs? Maybe, but I hope not? There would still be a primary owner of the memory, i.e. the memory would still need to be allocated in the context of a specific VM. And the primary owner should be able to restrict privileges, e.g. allow a different VM to read but not write memory. My current thinking is to (a) tie the lifetime of the backing pages to the inode, i.e. allow allocations to outlive the original VM, and (b) create a new file each time memory is shared/donated with a different VM (or other entity in the kernel). That should make it fairly straightforward to provide different permissions, e.g. track them per-file, and I think should also avoid the need to change the memslot binding logic since each VM would have it's own view/bindings. Copy+pasting a relevant snippet from a lengthier response in a different thread[*]: Conceptually, I think KVM should to bind to the file. The inode is effectively the raw underlying physical storage, while the file is the VM's view of that storage. Practically, I think that gives us a clean, intuitive way to handle intra-host migration. Rather than transfer ownership of the file, instantiate a new file for the target VM, using the gmem inode from the source VM, i.e. create a hard link. That'd probably require new uAPI, but I don't think that will be hugely problematic. KVM would need to ensure the new VM's guest_memfd can't be mapped until KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM (which would also need to verify the memslots/bindings are identical), but that should be easy enough to enforce. That way, a VM, its memslots, and its SPTEs are tied to the file, while allowing the memory and the *contents* of memory to outlive the VM, i.e. be effectively transfered to the new target VM. And we'll maintain the invariant that each guest_memfd is bound 1:1 with a single VM. As above, that should also help us draw the line between mapping memory into a VM (file), and freeing/reclaiming the memory (inode). There will be extra complexity/overhead as we'll have to play nice with the possibility of multiple files per inode, e.g. to zap mappings across all files when punching a hole, but the extra complexity is quite small, e.g. we can use address_space.private_list to keep track of the guest_memfd instances associated with the inode. Setting aside TDX and SNP for the moment, as it's not clear how they'll support memory that is "private" but shared between multiple VMs, I think per-VM files would work well for sharing gmem between two VMs. E.g. would allow a give page to be bound to a different gfn for each VM, would allow having different permissions for each file (e.g. to allow fallocate() only from the original owner). [*] https://lore.kernel.org/all/ZLGiEfJZTyl7M8mS@google.com _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv