Re: [PATCH v2] hostmem-file: add offset option

[Markus Armbruster]

Alexander Graf <graf@amazon.com> writes:

> Add an option for hostmem-file to start the memory object at an offset
> into the target file. This is useful if multiple memory objects reside
> inside the same target file, such as a device node.
>
> In particular, it's useful to map guest memory directly into /dev/mem
> for experimentation.
>
> Signed-off-by: Alexander Graf <graf@amazon.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index a877b879b9..8f5eaa8415 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -635,6 +635,10 @@
>  #         specify the required alignment via this option.
>  #         0 selects a default alignment (currently the page size). (default: 
> 0)
>  #
> +# @offset: the offset into the target file that the region starts at. You can
> +#          use this option to overload multiple regions into a single fils.

single file

I'm not sure about "to overload multiple regions into a single file".
Maybe "to back multiple regions with a single file".

Any alignment requirements?

What happens when the regions overlap?

> +#          (default: 0)

(since 8.1)

> +#
>  # @discard-data: if true, the file contents can be destroyed when QEMU exits,
>  #                to avoid unnecessarily flushing data to the backing file. 
> Note
>  #                that ``discard-data`` is only an optimization, and QEMU 
> might
> @@ -655,6 +659,7 @@
>  { 'struct': 'MemoryBackendFileProperties',
>    'base': 'MemoryBackendProperties',
>    'data': { '*align': 'size',
> +            '*offset': 'size',
>              '*discard-data': 'bool',
>              'mem-path': 'str',
>              '*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },

[...]