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

[Igor Mammedov]

On Tue, 28 Aug 2018 18:55:35 +0200
Marc-André Lureau <address@hidden> wrote:

> On Tue, Aug 28, 2018 at 6:54 PM Marc-André Lureau
> <address@hidden> wrote:
> >
> > 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.  
> 
> keeping the default value to shared=trrue.
ok,
another question:
 does it make sense to allow "shared=false" at all?
 should we bail out if user asks for it?

> 
> >  
> > >  
> > > >
> > > > 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  
> 
> 
>