Re: [PATCH 9/9] hostmem-file: support POSIX shm_open()

[Markus Armbruster]

Stefano Garzarella <sgarzare@redhat.com> writes:

> Add a new `shm` bool option for `-object memory-backend-file`.
>
> When this option is set to true, the POSIX shm_open(3) is used instead
> of open(2).
>
> So a file will not be created in the filesystem, but a "POSIX shared
> memory object" will be instantiated. In Linux this turns into a file
> in /dev/shm, but in other OSes this may not happen (for example in
> macOS or FreeBSD nothing is shown in any filesystem).
>
> This new feature is useful when we need to share guest memory with
> another process (e.g. vhost-user backend), but we don't have
> memfd_create() or any special filesystems (e.g. /dev/shm) available
> as in macOS.
>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> I am not sure this is the best way to support shm_open() in QEMU.
>
> Other solutions I had in mind were:
>
> - create a new memory-backend-shm

How would that look like?  Would it involve duplicating code?

> - extend memory-backend-memfd to use shm_open() on systems where memfd is
> not available (problem: shm_open wants a name to assign to the object, but
> we can do a workaround by using a random name and do the unlink right away)

Hmm.  Too much magic?  I don't know...

> Any preference/suggestion?

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index 2a6e49365a..bfb01b909f 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -682,6 +682,9 @@
   # @mem-path: the path to either a shared memory or huge page
   #     filesystem mount

Does this need adjustment?

[...]

>  #       writable RAM instead of ROM, and want to set this property to 'off'.
>  #       (default: auto, since 8.2)
>  #
> +# @shm: if true, shm_open(3) is used to create/open POSIX shared memory
> +#       object; if false, an open(2) is used. (default: false) (since 9.0)
> +#

Please format like this for consistency:

# @shm: if true, shm_open(3) is used to create/open POSIX shared memory
#     object; if false, an open(2) is used (default: false) (since 9.0)

>  # Since: 2.1
>  ##
>  { 'struct': 'MemoryBackendFileProperties',
> @@ -692,6 +695,7 @@
>              'mem-path': 'str',
>              '*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },
>              '*readonly': 'bool',
> +            '*shm': 'bool',
>              '*rom': 'OnOffAuto' } }
>  
>  ##

[...]