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

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jun 29, 2017 at 01:34:22PM +0000, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Jun 28, 2017 at 8:29 PM Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > 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?
> >
> >
> Actually, vhost-user may be the only way today to get the fd outside of
> qemu process.
> 
> (ivshmem needs the server to provide the fd)
> 
> We could quite easily add or extend QMP messages for that in the future.
> 
> Do you want this detail to be written in the commit message or elsewhere?

(Sorry for taking so long to reply.)

I think this should be in the documentation of the new option (in
qemu-options.hx).

I don't think it needs lots of extra detail, maybe just change "which
can be used to share the memory with a co-operating external process" to
"which allows QEMU to share the memory with an external process in some
cases (e.g. when using vhost-user)".

-- 
Eduardo