[libvirt] [RFC 0/7] Warn at runtime when deprecated features are used

[Andrea Bolognani]

On Fri, 2018-10-05 at 09:54 +0100, Daniel P. Berrangé wrote:
> On Fri, Oct 05, 2018 at 10:34:33AM +0200, Jiri Denemark wrote:
> > But what if QEMU (or any other hypervisor) marks something (device
> > model, machine type) as deprecated and we use that deprecated value as
> > our default. Shouldn't we be able to tell about it to our users (here
> > runtime warnings could help) when they use such thing explicitly and
> > choose a different default value? That would help users with the
> > transition and once hypervisor drops support for it completely, fewer
> > existing domains will be affected since the recently created ones would
> > already use non-deprecated defaults.
> 
> QEMU already emits warnings on stderr whenever something that is
> deprecated is used, and those end up in the libvirt log file for
> that guest. I don't think that we need to duplicate what QEMU
> is already reporting again.

Warnings printed on stderr -> users and developers will actually
see them, be annoyed by them, eventually cave in and act upon them.

Warnings written to a log -> nobody will notice them, until one day
things suddenly stop working apparently out of the blue.

We might pretend that's not the case, but really, it is.

> > So taking "q35" as an example, if QEMU marks i440fx machine types as
> > deprecated (they seem to be thinking about it) I don't see that big
> > difference[*] in changing the default machine type to "q35". After all
> > we will automatically change the default once i440fx is removed
> > completely.
> 
> There's no active plans to deprecate or remove i440fx upstream in
> QEMU.  If a downstream did remove it, then clearly that'll impose
> some level of pain on applications.

While i440fx itself has not been deprecated and might possibly
never be, other machine types (x86's pc-0.10 and pc-0.11, ppc's
prep) are in fact already marked as deprecated and will eventually
be dropped entirely from QEMU.

-- 
Andrea Bolognani / Red Hat / Virtualization