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

[Markus Armbruster <armbru@redhat.com>]

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 30e76653ad..d4e16c57a3 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -649,6 +649,7 @@
>  { 'struct': 'MemoryBackendFileProperties',
>    'base': 'MemoryBackendProperties',
>    'data': { '*align': 'size',
> +            '*offset': 'size',
>              '*discard-data': 'bool',
>              'mem-path': 'str',
>              '*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },

Looks like you forgot to document the new member.  You may want to ...

> diff --git a/qemu-options.hx b/qemu-options.hx
> index d59d19704b..2f12a9299e 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4875,7 +4875,7 @@ 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,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``
>          Creates a memory file backend object, which can be used to back
>          the guest RAM with huge pages.
>  
> @@ -4945,6 +4945,10 @@ SRST
>          such cases, users can specify the required alignment via this
>          option.
>  
> +        The ``offset`` option specifies the offset into the target file
> +        that the region starts at. You can use this parameter to overload
> +        multiple regions into a single file.
> +
>          The ``pmem`` option specifies whether the backing file specified
>          by ``mem-path`` is in host persistent memory that can be
>          accessed using the SNIA NVM programming model (e.g. Intel

... adapt the CLI documentation for it.

[...]