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

[Marc-André Lureau]

Hi

On Tue, Aug 28, 2018 at 6:49 PM Igor Mammedov <address@hidden> wrote:
>
> On Tue, 28 Aug 2018 17:38:40 +0200
> Marc-André Lureau <address@hidden> 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.

memfd backend is passing shared=true to memory_region_init_ram_from_fd().

The patch allows to tweak that (instead of ignoring the property),
while keeping the value to shared=true.

>
> >
> > Signed-off-by: Marc-André Lureau <address@hidden>
> > ---
> >  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.
> >
> > address@hidden -object 
> > memory-backend-memfd,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave},address@hidden|off},address@hidden|off},address@hidden
> > address@hidden -object 
> > memory-backend-memfd,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave},address@hidden|off},address@hidden|off},address@hidden
> >
> >  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,address@hidden,address@hidden/dev/random}
> >
> >  Creates a random number generator backend which obtains entropy from
>
>


-- 
Marc-André Lureau