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.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (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 4104DC4332F for ; Thu, 2 Nov 2023 16:04:43 +0000 (UTC) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=aU1idF4w; dkim-atps=neutral Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4SLpbs5xTQz3cnr for ; Fri, 3 Nov 2023 03:04:41 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=aU1idF4w; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=flex--seanjc.bounces.google.com (client-ip=2607:f8b0:4864:20::649; helo=mail-pl1-x649.google.com; envelope-from=3ymhdzqykddefrnawptbbtyr.pbzyvahkccp-qriyvfgf.bmynof.bet@flex--seanjc.bounces.google.com; receiver=lists.ozlabs.org) Received: from mail-pl1-x649.google.com (mail-pl1-x649.google.com [IPv6:2607:f8b0:4864:20::649]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4SLpZs4gfXz2xTm for ; Fri, 3 Nov 2023 03:03:47 +1100 (AEDT) Received: by mail-pl1-x649.google.com with SMTP id d9443c01a7336-1cc391ca417so9335295ad.0 for ; Thu, 02 Nov 2023 09:03:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698941024; x=1699545824; darn=lists.ozlabs.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=aU1idF4wvXgG9iFT3OhIKiYQd5c3QGnxWiuQq/Wl/VHcyEJfgLIorrp/hlbZC3XymU YdIBoEPnFgseCsz4URfsLUMbuXuFWv/uhNBKVqdlDt3272VchtazG8yk0KCvvhLVka1d ONy7lQE+rJaZegekNU0+qy/TfctpuLH9MO5Sg+DJdbsv76W3pjp1z0Gt7MYmOF/5W3Gj waFU/GJ7YtX7KZIY6p+Q3my7KHhN+PrPXsQ+EEA+4HpPSdgzxELo/ZexgmL143pGkLq0 mp8JAqHurianr/Mpa0QGPmmLkdeoXVnP11K3cl+UOkHOUXd62fxbbGHZc1W2jzlvRyoH zzxQ== 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=XnccxSXRXeyQrbPtdh3iXAkV6v128p8ncImEAzK597tY0BFQ49ynT8AMfjo8ZFG7ca HuIA5BDgAX0LmvodlXaj23nix8ZUHW1MRuGxx+d5yV2qMK2sUwWp/r9guyaaIT1rFZ2O 3dEtppae8gGWFsFX9srYoqKvidnGU4Y1hNVYMbj544IMoRzuLx6wRk7W60SvpznMQj4l ev1M4vvoIM5wMlQ5PTJ+t7cStFjjLHb5K8w/APxJbLsuKgdKzZdaAb4MRorhxJHDA1l1 JbeWhPuacJD/Ri2a4uZcwvF2vlhvn3jN6noxMxrL6EwOj6Ndhdf6X8cPQM4a7BxhNUml ensQ== X-Gm-Message-State: AOJu0YwcELuI1Cut9pPG+M5QK2kbIZfqn5zTGGP7r+6b8rzbEFOuKzMr ua0acQz/hBbGvV1Fddr+/O6aewNyWro= 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 Content-Type: text/plain; charset="us-ascii" X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kvm@vger.kernel.org, David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Chao Peng , linux-riscv@lists.infradead.org, Isaku Yamahata , Marc Zyngier , Huacai Chen , Xiaoyao Li , "Matthew Wilcox \(Oracle\)" , Wang , Fuad Tabba , Yu Zhang , Maciej Szmigiero , Albert Ou , Vlastimil Babka , Michael Roth , Ackerley Tng , Alexander Viro , Paul Walmsley , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , Isaku Yamahata , Christian Brauner , Quentin Perret , A nup Patel , linux-mips@vger.kernel.org, Oliver Upton , David Matlack , Jarkko Sakkinen , Palmer Dabbelt , "Kirill A . Shutemov" , kvm-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, Liam Merwick , Andrew Morton , Vishal Annapurve , linuxppc-dev@lists.ozlabs.org, Xu Yilun , Anish Moorthy Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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.