Re: device compatibility interface for live migration with assigned devices

[Sean Mooney <smooney@redhat.com>]

On Thu, 2020-07-30 at 11:41 +0800, Yan Zhao wrote:
> > > >    interface_version=3
> > 
> > Not much granularity here, I prefer Sean's previous
> > <major>.<minor>[.bugfix] scheme.
> > 
> 
> yes, <major>.<minor>[.bugfix] scheme may be better, but I'm not sure if
> it works for a complicated scenario.
> e.g for pv_mode,
> (1) initially,  pv_mode is not supported, so it's pv_mode=none, it's 0.0.0,
> (2) then, pv_mode=ppgtt is supported, pv_mode="none+ppgtt", it's 0.1.0,
> indicating pv_mode=none can migrate to pv_mode="none+ppgtt", but not vice versa.
> (3) later, pv_mode=context is also supported,
> pv_mode="none+ppgtt+context", so it's 0.2.0.
> 
> But if later, pv_mode=ppgtt is removed. pv_mode="none+context", how to
> name its version?
it would become 1.0.0
addtion of a feature is a minor version bump as its backwards compatiable.
if you dont request the new feature you dont need to use it and it can continue to behave like
a 0.0.0 device evne if its capably of acting as a 0.1.0 device.
when you remove a feature that is backward incompatable as any isnstance that was prevously not
using it would nolonger work so you have to bump the major version.
>  "none+ppgtt" (0.1.0) is not compatible to
> "none+context", but "none+ppgtt+context" (0.2.0) is compatible to
> "none+context".
> 
> Maintain such scheme is painful to vendor driver.
not really its how most software libs are version today. some use other schemes
but semantic versioning is don right is a concies and easy to consume set of rules
https://semver.org/ however you are right that it forcnes vendor to think about backwards
and forwards compatiablty with each change which for the most part is a good thing.
it goes hand in hand with have stable abi and api definitons to ensuring firmware updates and driver chagnes
dont break userspace that depend on the kernel interfaces they expose.