Re: [PATCH RESEND v7 10/12] hostmem: add a new memory backend based on POSIX shm_open()

[Markus Armbruster]

Stefano Garzarella <sgarzare@redhat.com> writes:

> shm_open() creates and opens a new POSIX shared memory object.
> A POSIX shared memory object allows creating memory backend with an
> associated file descriptor that can be shared with external processes
> (e.g. vhost-user).
>
> The new `memory-backend-shm` can be used as an alternative when
> `memory-backend-memfd` is not available (Linux only), since shm_open()
> should be provided by any POSIX-compliant operating system.
>
> This backend mimics memfd, allocating memory that is practically
> anonymous. In theory shm_open() requires a name, but this is allocated
> for a short time interval and shm_unlink() is called right after
> shm_open(). After that, only fd is shared with external processes
> (e.g., vhost-user) as if it were associated with anonymous memory.
>
> In the future we may also allow the user to specify the name to be
> passed to shm_open(), but for now we keep the backend simple, mimicking
> anonymous memory such as memfd.
>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index 9b8f6a7ab5..94e4458288 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -601,8 +601,8 @@
>  #
>  # @share: if false, the memory is private to QEMU; if true, it is
>  #     shared (default false for backends memory-backend-file and
> -#     memory-backend-ram, true for backends memory-backend-epc and
> -#     memory-backend-memfd)
> +#     memory-backend-ram, true for backends memory-backend-epc,
> +#     memory-backend-memfd, and memory-backend-shm)
>  #
>  # @reserve: if true, reserve swap space (or huge pages) if applicable
>  #     (default: true) (since 6.1)
> @@ -721,6 +721,22 @@
>              '*hugetlbsize': 'size',
>              '*seal': 'bool' } }
>  
> +##
> +# @MemoryBackendShmProperties:
> +#
> +# Properties for memory-backend-shm objects.
> +#
> +# Setting @share boolean option (defined in the base type) to false
> +# will cause a failure during allocation because it is not
> +# supported by this backend.

This is QMP reference documentation.  "Failure during allocation" feels
like unnecessary detail there.  Maybe "This memory backend support only
shared memory, which is the default."

> +#
> +# Since: 9.1
> +##
> +{ 'struct': 'MemoryBackendShmProperties',
> +  'base': 'MemoryBackendProperties',
> +  'data': { },
> +  'if': 'CONFIG_POSIX' }
> +
>  ##
>  # @MemoryBackendEpcProperties:
>  #
> @@ -1049,6 +1065,8 @@
>      { 'name': 'memory-backend-memfd',
>        'if': 'CONFIG_LINUX' },
>      'memory-backend-ram',
> +    { 'name': 'memory-backend-shm',
> +      'if': 'CONFIG_POSIX' },
>      'pef-guest',
>      { 'name': 'pr-manager-helper',
>        'if': 'CONFIG_LINUX' },
> @@ -1121,6 +1139,8 @@
>        'memory-backend-memfd':       { 'type': 'MemoryBackendMemfdProperties',
>                                        'if': 'CONFIG_LINUX' },
>        'memory-backend-ram':         'MemoryBackendProperties',
> +      'memory-backend-shm':         { 'type': 'MemoryBackendShmProperties',
> +                                      'if': 'CONFIG_POSIX' },
>        'pr-manager-helper':          { 'type': 'PrManagerHelperProperties',
>                                        'if': 'CONFIG_LINUX' },
>        'qtest':                      'QtestProperties',

[...]

Other than that, QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>