From e92666b0ba4cbaa71a5dd98c31414926a9915487 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 6 Sep 2023 14:04:55 +0200 Subject: backends/hostmem-file: Add "rom" property to support VM templating with R/O files 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. Message-ID: <20230906120503.359863-4-david@redhat.com> Acked-by: Markus Armbruster Signed-off-by: David Hildenbrand --- qemu-options.hx | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) (limited to 'qemu-options.hx') diff --git a/qemu-options.hx b/qemu-options.hx index 6be621c232..3aabd6e0cc 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4995,7 +4995,7 @@ SRST they are specified. Note that the 'id' property must be set. These objects are placed in the '/objects' path. - ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,offset=offset,readonly=on|off`` + ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,offset=offset,readonly=on|off,rom=on|off|auto`` Creates a memory file backend object, which can be used to back the guest RAM with huge pages. @@ -5085,6 +5085,20 @@ SRST The ``readonly`` option specifies whether the backing file is opened read-only or read-write (default). + The ``rom`` option specifies whether to create Read Only Memory + (ROM) that cannot be modified by the VM. Any write attempts to such + ROM will be denied. Most use cases want proper RAM instead of ROM. + However, selected use cases, like R/O NVDIMMs, can benefit from + ROM. If set to ``on``, create ROM; if set to ``off``, create + writable RAM; if set to ``auto`` (default), the value of the + ``readonly`` option is used. This option is primarily helpful when + we want to have writable RAM in configurations that would + traditionally create ROM before the ``rom`` option was introduced: + VM templating, where we want to open a file readonly + (``readonly=on``) and mark the memory to be private for QEMU + (``share=off``). For this use case, we need writable RAM instead + of ROM, and want to also set ``rom=off``. + ``-object memory-backend-ram,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave`` Creates a memory backend object, which can be used to back the guest RAM. Memory backend objects offer more control than the -- cgit v1.2.3