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Tsirkin" , Ani Sinha , Xiao Guangrong , Daniel Henrique Barboza , Greg Kurz , Eric Blake , Eduardo Habkost Subject: Re: [PATCH v3 03/11] backends/hostmem-file: Add "rom" property to support VM templating with R/O files References: <20230823153412.832081-1-david@redhat.com> <20230823153412.832081-4-david@redhat.com> Date: Fri, 01 Sep 2023 14:50:17 +0200 In-Reply-To: <20230823153412.832081-4-david@redhat.com> (David Hildenbrand's message of "Wed, 23 Aug 2023 17:34:03 +0200") Message-ID: <87y1hqxdkm.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org David Hildenbrand writes: > For now, "share=off,readonly=on" would always result in us opening the > file R/O and mmap'ing the opened file MAP_PRIVATE R/O -- effectively > turning it into ROM. > > Especially for VM templating, "share=off" is a common use case. However, > that use case is impossible with files that lack write permissions, > because "share=off,readonly=on" will not give us writable RAM. > > The sole user of ROM via memory-backend-file are R/O NVDIMMs, but as we > have users (Kata Containers) that rely on the existing behavior -- > malicious VMs should not be able to consume COW memory for R/O NVDIMMs -- > we cannot change the semantics of "share=off,readonly=on" > > So let's add a new "rom" property with on/off/auto values. "auto" is > the default and what most people will use: for historical reasons, to not > change the old semantics, it defaults to the value of the "readonly" > property. > > For VM templating, one can now use: > -object memory-backend-file,share=off,readonly=on,rom=off,... > > But we'll disallow: > -object memory-backend-file,share=on,readonly=on,rom=off,... > because we would otherwise get an error when trying to mmap the R/O file > shared and writable. An explicit error message is cleaner. > > We will also disallow for now: > -object memory-backend-file,share=off,readonly=off,rom=on,... > -object memory-backend-file,share=on,readonly=off,rom=on,... > It's not harmful, but also not really required for now. > > Alternatives that were abandoned: > * Make "unarmed=on" for the NVDIMM set the memory region container > readonly. We would still see a change of ROM->RAM and possibly run > into memslot limits with vhost-user. Further, there might be use cases > for "unarmed=on" that should still allow writing to that memory > (temporary files, system RAM, ...). > * Add a new "readonly=on/off/auto" parameter for NVDIMMs. Similar issues > as with "unarmed=on". > * Make "readonly" consume "on/off/file" instead of being a 'bool' type. > This would slightly changes the behavior of the "readonly" parameter: > values like true/false (as accepted by a 'bool'type) would no longer be > accepted. > > Signed-off-by: David Hildenbrand QAPI schema Acked-by: Markus Armbruster