Re: [Qemu-devel] [PATCH] hostmem-memfd: honour share=on/off property

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 28 Aug 2018 17:38:40 +0200
Marc-André Lureau <marcandre.lureau@redhat.com> wrote:

> The share=on/off property is used to modified mmap() MAP_SHARED
> setting. Make it on by default for convenience and compatibility
> reasons.
it would be nice to state reasons here.

also flipping default to on is behavioral change when
used with bind policy which could make backend initialization fail
so CLI that used to work (if there are any users of this without shared=on)
won't work anymore.

> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  backends/hostmem-memfd.c | 4 +++-
>  qemu-options.hx          | 4 +++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> index 1e20fe0ba8..abd52364db 100644
> --- a/backends/hostmem-memfd.c
> +++ b/backends/hostmem-memfd.c
> @@ -59,7 +59,8 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>  
>      name = object_get_canonical_path(OBJECT(backend));
>      memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
> -                                   name, backend->size, true, fd, errp);
> +                                   name, backend->size,
> +                                   backend->share, fd, errp);
>      g_free(name);
>  }
>  
> @@ -131,6 +132,7 @@ memfd_backend_instance_init(Object *obj)
>  
>      /* default to sealed file */
>      m->seal = true;
> +    MEMORY_BACKEND(m)->share = true;
>  }
>  
>  static void
> diff --git a/qemu-options.hx b/qemu-options.hx
> index f14e423b7c..25c02d72b8 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4029,7 +4029,7 @@ Memory backend objects offer more control than the @option{-m} option that is
>  traditionally used to define guest RAM. Please refer to
>  @option{memory-backend-file} for a description of the options.
>  
> -@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
> +@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
>  
>  Creates an anonymous memory file backend object, which allows QEMU to
>  share the memory with an external process (e.g. when using
> @@ -4051,6 +4051,8 @@ with the @option{seal} option (requires at least Linux 4.16).
>  Please refer to @option{memory-backend-file} for a description of the
>  other options.
>  
> +The @option{share} boolean option is @var{on} by default with memfd.
> +
>  @item -object rng-random,id=@var{id},filename=@var{/dev/random}
>  
>  Creates a random number generator backend which obtains entropy from