Re: [PATCH] net: improve error message for missing netdev backend

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 3. Oktober 2022 14:50:04 CEST Daniel P. Berrangé wrote:
> On Mon, Oct 03, 2022 at 02:46:04PM +0200, Christian Schoenebeck wrote:
> > On Montag, 3. Oktober 2022 12:06:12 CEST Daniel P. Berrangé wrote:
> > > The current message when using '-net user...' with SLIRP disabled at
> > > 
> > > compile time is:
> > >   qemu-system-x86_64: -net user: Parameter 'type' expects a net backend
> > >   type
> > > 
> > > (maybe it is not compiled into this binary)
> > 
> > Is this intended as alternative to Marc-André's previous patch?
> 
> This is a patch that should be applied regardless of any other change,
> because the error message we report here today is awful and needs
> improving.
> 
> >                                                                  If yes,
> >                                                                  then
> > 
> > same applies here: what about people not passing any networking arg to
> > QEMU? They would not get any error message at all, right?
> 
> Yes, I mentioned that in the text that you've quoted below....

Yeah, missed that one, sorry.

> > > An observation is that we're using the 'netdev->type' field here which
> > > is an enum value, produced after QAPI has converted from its string
> > > form.
> > > 
> > > IOW, at this point in the code, we know that the user's specified
> > > type name was a valid network backend. The only possible scenario that
> > > can make the backend init function be NULL, is if support for that
> > > backend was disabled at build time. Given this, we don't need to caveat
> > > our error message with a 'maybe' hint, we can be totally explicit.
> > > 
> > > The use of QERR_INVALID_PARAMETER_VALUE doesn't really lend itself to
> > > user friendly error message text. Since this is not used to set a
> > > specific QAPI error class, we can simply stop using this pre-formatted
> > > error text and provide something better.
> > > 
> > > Thus the new message is:
> > >   qemu-system-x86_64: -net user: network backend 'user' is not compiled
> > >   into
> > > 
> > > this binary
> > 
> > And why not naming the child, i.e. that QEMU was built without slirp?
> 
> There are several network backends that can be conditionally disabled
> at build time, and IMHO its overkill to give a different message for
> each one. This message is sufficient to show users where to go next.

Yes, but that is not a user friendly error message, especially for people who 
never dealt with QEMU's networking options before. That message does not make 
it obvious how to find the solution IMO.

What about a web link to the QEMU networking docs where this issue could then 
be clarified in a more user friendly manner? #anchors_are_cheap

> > > The case of passing 'hubport' for -net is also given a message reminding
> > > people they should have used -netdev/-nic instead, as this backend type
> > > is only valid for the modern syntax.
> > > 
> > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > > ---
> > > 
> > > NB, this does not make any difference to people who were relying on the
> > > QEMU built-in default hub that was created if you don't list any -net /
> > > -netdev / -nic argument, only those using explicit args.
> 
> .... here.
> 
> 
> 
> With regards,
> Daniel