Re: [PATCH v2 3/9] backends/hostmem-file: Add "rom" property to support VM templating with R/O files

[Markus Armbruster <armbru@redhat.com>]

David Hildenbrand <david@redhat.com> 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 <david@redhat.com>

[...]

>  static void file_backend_instance_finalize(Object *o)
> diff --git a/qapi/qom.json b/qapi/qom.json
> index fa3e88c8e6..0cf83c6f39 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -668,6 +668,9 @@
>  # @readonly: if true, the backing file is opened read-only; if false,
>  #     it is opened read-write.  (default: false)
>  #
> +# @rom: whether to create Read Only Memory (ROM).  If set to 'auto', it
> +#       defaults to the value of @readonly.  (default: auto, since 8.2)
> +#
>  # Since: 2.1
>  ##

The commit message discusses how @readonly, @rom and @share interact.
The doc comments don't, and users have to guess.

I can see two ways to help users:

1. Describe their interaction in full, so users can understand how to
get from them what they need.

2. Provide suitable guidance on how to use them.

>  { 'struct': 'MemoryBackendFileProperties',
> @@ -677,7 +680,8 @@
>              '*discard-data': 'bool',
>              'mem-path': 'str',
>              '*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },
> -            '*readonly': 'bool' } }
> +            '*readonly': 'bool',
> +            '*rom': 'OnOffAuto' } }
>  ##
>  # @MemoryBackendMemfdProperties:
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 29b98c3d4c..03ce0b0a30 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4976,7 +4976,7 @@ 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.
>  
> @@ -5066,6 +5066,14 @@ 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. If set to ``on``, the VM cannot
> +        modify the memory. If set to ``off``, the VM can modify the memory.
> +        If set to ``auto`` (default), the value of the ``readonly`` property
> +        is used. This option is primarily helpful for VM templating, where we
> +        want to open a file readonly (``readonly=on``) and allow private
> +        modifications of the memory by the VM (``share=off``, ``rom=off``).
> +

Here, you provide some guidance.

>      ``-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