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

[Stefano Garzarella]

On Wed, Jun 12, 2024 at 03:20:48PM GMT, Markus Armbruster wrote:
> 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."

I'll fix in v8!

> > +#
> > +# 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>

Thanks for the review!
Stefano