Re: [PATCH 2/3] docs/s390x: document 3270

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 15 May 2020 09:29:42 +0200
Thomas Huth <thuth@redhat.com> wrote:

> On 05/05/2020 15.50, Cornelia Huck wrote:
> > Add some basic info how to use 3270 devices.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> >  docs/system/s390x/3270.rst   | 32 ++++++++++++++++++++++++++++++++
> >  docs/system/target-s390x.rst |  1 +
> >  2 files changed, 33 insertions(+)
> >  create mode 100644 docs/system/s390x/3270.rst
> > 
> > diff --git a/docs/system/s390x/3270.rst b/docs/system/s390x/3270.rst
> > new file mode 100644
> > index 000000000000..e367a457e001
> > --- /dev/null
> > +++ b/docs/system/s390x/3270.rst
> > @@ -0,0 +1,32 @@
> > +3270 devices
> > +============
> > +
> > +With the aid of the ``x3270`` emulator, QEMU provides limited support
> > +for making a single 3270 device available to a guest. Note that this
> > +supports basic features only.  
> 
> The first sentence sounds somewhat confusing. Maybe rather something like:
> 
> QEMU can emulate a 3270 device attached to a guest, which then can be
> used with a program like ``x3270`` to get a traditional 3270 terminal
> for your guest.

But QEMU actually relies on x3270 (or a comparable program). It only
emulates the basic ccw plumbing; for the actual protocol (beyond
negotiating tn3270), it relies on the emulation done by x3270.

> 
> ?
> 
> > +To provide a 3270 device to a guest, create a ``x-3270device`` linked to  
> 
> The device is called "x-terminal3270", isn't it?

Yeah, no idea how I manged to mess that up.

> 
> (By the way, why did we never remove the x- prefix here? Is it still
> experimental?)

I'd still consider it experimental: only basic functionality is
supported, and (more importantly), we never came up with a scheme to
properly support more than one 3270 device. The usefulness of it is
quite limited.

> 
> > +a ``tn3270`` chardev. The guest will see a 3270 channel device. In order
> > +to actually be able to use it, attach the ``x3270`` emulator to the chardev.  
> 
> Can the guest only use the 3270 device if there is a x3270 attached to
> it? Or are you talking about the user here?

See above: x3270 does most of the heavy lifting.

> 
> > +Example configuration
> > +---------------------
> > +
> > +* Add a ``tn3270`` chardev and a ``x-3270device`` to the QEMU command line::  
> 
> "x-terminal3270" ?

Nod.

> 
> > +    -chardev socket,id=char_0,host=0.0.0.0,port=23,nowait,server,tn3270
> > +    -device x-terminal3270,chardev=char_0,devno=fe.0.000a,id=terminal_0  
> 
> qemu-system-s390x: -chardev
> socket,id=char_0,host=0.0.0.0,port=23,nowait,server,tn3270: Failed to
> bind socket: Permission denied
> 
> ... maybe better use a non-privileged port in the example?

Makes sense. This example was lifted straight from the wiki page :)