Re: docs: Update vhost-user spec regarding backend program conventions

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Feb 14, 2020 at 03:00:34PM +0100, Marc-André Lureau wrote:
> Hi
> 
> On Fri, Feb 14, 2020 at 2:24 PM Boeuf, Sebastien
> <sebastien.boeuf@intel.com> wrote:
> >
> > Hi Marc-Andre,
> >
> > On Tue, 2020-02-11 at 22:24 +0100, Marc-André Lureau wrote:
> > > Hi
> > >
> > > On Tue, Feb 11, 2020 at 4:24 PM Boeuf, Sebastien
> > > <sebastien.boeuf@intel.com> wrote:
> > > > From c073d528b8cd7082832fd1825dc33dd65b305aa2 Mon Sep 17 00:00:00
> > > > 2001
> > > > From: Sebastien Boeuf <sebastien.boeuf@intel.com>
> > > > Date: Tue, 11 Feb 2020 16:01:22 +0100
> > > > Subject: [PATCH] docs: Update vhost-user spec regarding backend
> > > > program
> > > >  conventions
> > > >
> > > > The vhost-user specification is not clearly stating the expected
> > > > behavior from a backend program whenever the client disconnects.
> > > >
> > > > This patch addresses the issue by defining the default behavior and
> > > > proposing an alternative through a command line option.
> > > >
> > > > By default, a backend program will have to keep listening even if
> > > > the
> > > > client disconnects, unless told otherwise through the newly
> > > > introduced
> > > > option --exit-on-disconnect.
> > > >
> > > > Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
> > > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > > ---
> > > >  docs/interop/vhost-user.rst | 10 ++++++++++
> > > >  1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-
> > > > user.rst
> > > > index 5f8b3a456b..da9a1ebc4c 100644
> > > > --- a/docs/interop/vhost-user.rst
> > > > +++ b/docs/interop/vhost-user.rst
> > > > @@ -1323,6 +1323,10 @@ The backend program must end (as quickly and
> > > > cleanly as possible) when
> > > >  the SIGTERM signal is received. Eventually, it may receive SIGKILL
> > > > by
> > > >  the management layer after a few seconds.
> > > >
> > > > +By default, the backend program continues running after the client
> > > > +disconnects. It accepts only 1 connection at a time on each UNIX
> > > > domain
> > > > +socket.
> > >
> > > I don't think that's the most common behaviour. libvhost-user will
> > > panic() on disconnect in general, unless the error/exit is handled
> > > gracefully by the backend.
> >
> > It's not the default behavior from libvhost-user, but that's exactly
> > something I'd like to see changing. This should be the normal case if
> > you think about a standard client/server connection, where the server
> > would simply listen again after the client disconnects.
> 
> I disagree, a "normal" lifecycle is a single connection & instance per device.
> 
> Having the backend handle multiple connections is needlessly more
> complicated. You need to correctly handle multiple states, flushed
> anything private between connections etc. It should be optional.
> 
> 
> > >
> > > The most common case is to have 1-1 relation between device/qemu
> > > instance and backend.
> >
> > Yes this part is fine, but that's not a reason why the backend should
> > terminates.
> 
> It is simpler to ensure it is reset correctly.
> 
> >
> > >
> > > Why not restart the backend for another instance?
> >
> > Because you need some management tool to do so. And I think that by
> > default it could be interesting to have the least amount of extra
> > management involved.
> 
> The management layer should be involved if any side crashes or restart anyway.

Further, this vhost-user.rst spec document is explicitly describing
the contract between the vhost-user binaries and the management
layer. So it doesn't make sense to say update this doc to describe
desired semantics for usage /without/ a management layer.

So I agree, the default behaviour should be that there is one binary
spawned at time the associated device is initialization, and that
the lifetime of the binary is 1:1 associated with the lifetime of
the VM, or until the device is unplugged. 

Regards,
Daniel
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