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Re: [libvirt] [PATCH v2] vhost-user: define conventions for vhost-user backends



Hi

On Wed, Oct 31, 2018 at 8:05 PM Daniel P. Berrangé <berrange redhat com> wrote:
>
> On Wed, Oct 31, 2018 at 07:44:36PM +0400, Marc-André Lureau wrote:
> > Hi
> >
> > On Tue, Oct 16, 2018 at 4:14 PM Daniel P. Berrangé <berrange redhat com> wrote:
> > > > > It just reinvents the chardev unix socket syntax, but in a
> > > > > different adhoc manner, which is repeating the mistake we have
> > > > > made time & again in QEMU. Using QAPI we can directly accept
> > > > > the ChardevSocket syntax we already know about. Instead of
> > > > > having --socket-path and --fd and documenting that they are
> > > > > mutually exclusive ChardevSocket QAPI schema provides that
> > > > > representation in a well defined format which is discoverable
> > > > > and QEMU and mgmt apps already understand.
> > > >
> > > > That would require external vhost-user backends to implement QAPI/json
> > > > parsing. Is this necessary for a vhost-user backend? I doubt it.
> > >
> > > They could, but would not be required, to implement QAPI/json parser.
> > >
> > > The QAPI schema defines a standard for how to model & interpret the
> > > non-scalar values for command line arguments. An external impl would
> > > need to ensure that whatever parsing it does for CLI args is semantically
> > > compatible with the parsing rules defined by the QEMU QAPI schema we
> > > define to ensure interoperability of its impl.
> >
> > So you would want to have something like?
> >
> > --chardev '{ "id" : "bar", "backend" : { "type" : "socket", "data" : {
> > "addr" : { "type": "unix", "path": "/tmp/foo.sock" }, "server":
> > "false"  } } }'
>
> I wasn't specificially suggesting json syntax. Just the flattened
> dot separate syntax, whose valid components are mapped to corresponding
> qapi schema defintions. eg closer to what we have already today
>
>   --chardev socket,id=bar,path=/tmp/foo.sock,server
>

ok

> >
> > instead of:
> >
> > --socket-path=/tmp/foo.sock
> >
> > I don't really get what that will help with, for the vhost-user backend case...
>
> It avoids presuming that we forever want to launch the backend with
> a single socket path and nothing else. Using the chardev, means we
> can choosen between client/server mode. We can choose whether to
> pass an FD to the socket, instead of the socket path. Whether the
> reconnect flag is set or not, and all the other aspects of a chardev
> you can define.
>

We are trying to define a common specification for vhost-user
backends. Where do we stop defining what --chardev should support?

> I don't think we should have to add more & more adhoc CLI args (--socket-path,
> --fd, --reconnect, etc) and then manually parse them & pack their values
> together into a chardev object.

The backends most likely won't use qemu chardev (nor qapi), and be
limited to unix socket.


>
>
> Regards,
> Daniel
> --
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