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 E1209C4332F for ; Thu, 2 Nov 2023 16:04:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377099AbjKBQDv (ORCPT ); Thu, 2 Nov 2023 12:03:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49844 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234506AbjKBQDs (ORCPT ); Thu, 2 Nov 2023 12:03:48 -0400 Received: from mail-pl1-x649.google.com (mail-pl1-x649.google.com [IPv6:2607:f8b0:4864:20::649]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 103C512E for ; Thu, 2 Nov 2023 09:03:45 -0700 (PDT) Received: by mail-pl1-x649.google.com with SMTP id d9443c01a7336-1cc281f1214so9107485ad.2 for ; Thu, 02 Nov 2023 09:03:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698941024; x=1699545824; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=2iqXbuRKMyvTfyAi0R6oHncN26PfYTo3tctDI41vJWU=; b=TJNOze6kt+FsA9v0I74BIpCjw9fge82sCtaK3cxtT6c0KZEdKSzBh4zG31U+RQGN7+ RBc5D+JpR6UnV3KagOIZh/k0QWAS7g+80tHuxJWkMkRcptC9bydUspTt+ACMT2HAase2 fXKjWpOYXDOWK/pGCUZdZDZUFeL6l0Um/28DOwX4QiyMEnKher3A3Rm80G3S+GZA8zOu 8WeQaicXLIC7Cclvl/uP6/detJrBYWN3Khg0/VrDjV7a/4uhH35JM2byF4Fn7t6fjYJG lcIyR8iLWXR3b1X3wXSx08815FLcwy3qN//DPxL2Py0ry4RP/UvDUbRELdcVSBVZTg4/ g/Gw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698941024; x=1699545824; 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=2iqXbuRKMyvTfyAi0R6oHncN26PfYTo3tctDI41vJWU=; b=YXSNODTmPFuW/OAKm0LjdE/Ts8mWTK1U8JRC6xAdpVuYl4Rou2l1jJQgYLfnNZKD6O GKB38Wbfkh2ykoYJeVYpf6ZokemF2Q3YpbJabvR5g4kztFaHAqJ1mtpWBEZGzDW9v1Kq RvvqIRIEKXU/So7HsxQpb7uOWR2JRaKyD3LOC6MX8tWTKPHnjuJu6hRM+sBpFHdHTVkd FeL6HiYpD4+mBPNcUNo4p4KHGaAGsWZTBmZsxUo9BrFyBudMbLDuEy6VgCkjQfGVvBxb nLF8W6xS+86Dt0nzp/BpE9fBag2Hk6Hzv2E6WGCuKvq4ZtfI6wvcAnHFg8vke+UKevJV Qfzw== X-Gm-Message-State: AOJu0Yx5DmhoUZHSMdymvATyzabXpX6ABJYSgFc9DOI0uljyOUZJHovD ThUz+Mbn1Uk305P//z59sooPhR1+oYg= X-Google-Smtp-Source: AGHT+IFM1Nz2eAP9Xmws9JoQjAGGS19Q55KF/ifRxAflcrAzQHFBFIOS8V+26ERzfv1Lm5CG2bErCCPJO0Q= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:903:1304:b0:1cc:2ffe:5a33 with SMTP id iy4-20020a170903130400b001cc2ffe5a33mr266680plb.8.1698941024519; Thu, 02 Nov 2023 09:03:44 -0700 (PDT) Date: Thu, 2 Nov 2023 09:03:42 -0700 In-Reply-To: <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-17-seanjc@google.com> <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Message-ID: Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory From: Sean Christopherson To: Paolo Bonzini Cc: David Matlack , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexander Viro , Christian Brauner , "Matthew Wilcox (Oracle)" , Andrew Morton , 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-kernel@vger.kernel.org, Xiaoyao Li , Xu Yilun , Chao Peng , Fuad Tabba , Jarkko Sakkinen , Anish Moorthy , Yu Zhang , Isaku Yamahata , "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" , Vlastimil Babka , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata , "Kirill A . Shutemov" Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 02, 2023, Paolo Bonzini wrote: > On 10/31/23 23:39, David Matlack wrote: > > > > Maybe can you sketch out how you see this proposal being extensible to > > > > using guest_memfd for shared mappings? > > > For in-place conversions, e.g. pKVM, no additional guest_memfd is needed. What's > > > missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to > > > ensure there are no outstanding references when converting back to private. > > > > > > For TDX/SNP, assuming we don't find a performant and robust way to do in-place > > > conversions, a second fd+offset pair would be needed. > > Is there a way to support non-in-place conversions within a single guest_memfd? > > For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest > memory. The hook would invalidate now-private parts if they have a VMA, > causing a SIGSEGV/EFAULT if the host touches them. > > It would forbid mappings from multiple gfns to a single offset of the > guest_memfd, because then the shared vs. private attribute would be tied to > the offset. This should not be a problem; for example, in the case of SNP, > the RMP already requires a single mapping from host physical address to > guest physical address. I don't see how this can work. It's not a M:1 scenario (where M is multiple gfns), it's a 1:N scenario (wheren N is multiple offsets). The *gfn* doesn't change on a conversion, what needs to change to do non-in-place conversion is the pfn, which is effectively the guest_memfd+offset pair. So yes, we *could* support non-in-place conversions within a single guest_memfd, but it would require a second offset, at which point it makes sense to add a second file descriptor as well. Userspace could still use a single guest_memfd instance, i.e. pass in the same file descriptor but different offsets.