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Re: [libvirt] [PATCH] Drop UML driver

On Fri, Dec 14, 2018 at 03:46:16PM +0100, Michal Privoznik wrote:
> On 12/14/18 3:35 PM, Daniel P. Berrangé wrote:
> > On Fri, Dec 14, 2018 at 03:30:17PM +0100, Michal Privoznik wrote:
> >> The driver is unmaintained, untested and severely broken for
> >> quite some time now. Since nobody even reported any issue with it
> >> let us drop it.
> >>
> >> Signed-off-by: Michal Privoznik <mprivozn redhat com>
> >> ---
> > 
> >>  docs/schemas/capability.rng          |    2 -
> >>  docs/schemas/domaincommon.rng        |    3 -
> > 
> > 
> >>  src/conf/domain_conf.c               |   11 +-
> >>  src/conf/domain_conf.h               |    4 -
> > 
> > We shouldn't be deleting stuff from the XML schemas. IMHO the schemas
> > are an append only source object. If some parts happen to not be used
> > by current code that's fine, but they are a record of the ABI promise
> > of the schema.
> So we should be able to validate <domain type="uml"/> even though there
> is no longer any driver that would define such domain? I don't see much
> point in that.

The point is that the schema definition is independent of the driver
implementations. Implementations in libvirt come & go, but the schema
that they adhere to must remain constant.

> Also, removing a driver is breaking the ABI promise.

To some extent, but I don't consider that equivalent to the promise of
stability of our library ELF API or XML schema. Implementations of
a feature may have a finite lifetime. The way a feature is described
remains the same forever, which is what the XML schema declares. As
such its inappropriate to remove something from the schema, just
because the feature doesn't exist.

This can affect downstream applications, even if they are not actively
using the UML driver. For example libraries that provide an API around
our XML schema may be validating their implementation against our RNG
schemas & thus removing it can break those impls.

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