Re: [Qemu-devel] [PATCH v4 2/3] Add memfd based hostmem

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jun 27, 2017 at 08:23:03AM +0000, Marc-André Lureau wrote:
> Hi Eduardo
> 
> On Fri, Jun 23, 2017 at 11:09 PM Eduardo Habkost <ehabkost@redhat.com>
> wrote:
> 
> > On Wed, Jun 21, 2017 at 04:02:18PM +0200, Marc-André Lureau wrote:
> > > Add a new memory backend, similar to hostmem-file, except that it
> > > doesn't need to create files. It also enforces memory sealing.
> > >
> > > This backend is mainly useful for sharing the memory with other
> > > processes.
> >
> > How exactly can the memfd be used to share memory?  Is there an existing
> > mechanism for sharing the memfd file descriptor with another process
> 
> 
> 
> Since there is no backing file, the traditional mechanism is by passing fd,
> via socket ancillary data or forking etc.. Both ivshmem and vhost-user have
> such messages, with eventually details for the memory map usage.

The documentation is very similar to memory-backend-file, so it sounded
like there was a generic mechanism to ask QEMU to share the backend FD.
Maybe it would be interesting to mention on which cases the FD can
actually be shared.  Are ivshmem and vhost-user the only existing cases?

> 
> 
> > >
> > > Note that Linux supports transparent huge-pages of shmem/memfd memory
> > > since 4.8. It is relatively easier to set up THP than a dedicate
> > > hugepage mount point by using "madvise" in
> > > /sys/kernel/mm/transparent_hugepage/shmem_enabled.
> > >
> > > Usage:
> > > -object memory-backend-memfd,id=mem1,size=1G
> > >
> > > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > ---
> > >  backends/hostmem-memfd.c | 67
> > ++++++++++++++++++++++++++++++++++++++++++++++++
> > >  backends/Makefile.objs   |  2 ++
> > >  qemu-options.hx          | 11 ++++++++
> > >  3 files changed, 80 insertions(+)
> > >  create mode 100644 backends/hostmem-memfd.c
> > >
> > [...]
> >
> > --
> > Eduardo
> >
> > --
> Marc-André Lureau

-- 
Eduardo