Re: [Qemu-devel] Re: [PATCH] Revive -version 'QEMU PC Emulator...'

[Anthony Liguori]

On 05/14/2010 10:02 AM, Daniel P. Berrange wrote:
> On Fri, May 14, 2010 at 09:52:53AM -0500, Anthony Liguori wrote:
>    
> > On 05/14/2010 09:42 AM, Daniel P. Berrange wrote:
> >      
> > > It is preferable to query the explicit capability wanted, because
> > > version numbers are useless when distros backport features,
> > >        
> > Unless distros add their own release number to the version information
> > and libvirt learns about the features they add.
> >      
> And features removed / compiled out. eg SDL removed, es1370 sound driver
> removed, etc, etc.
>
> Version numbers are a really bad way to determine availability of
> functionality. You end up having to build a giant matrix of features
> vs version numbers, which will be outdated the moment its created.
>    

You need version numbers to understand behavior.  If you also want to 
know compile settings, we need the equivalent of /proc/config (a simple 
-build-config command could output our build config).

> If you want feature X, you need to be able to check for prescense of
> feature X, not infer it from another piece of information that is
> only loosely related.
>    

You assume that feature X is idempotent across multiple downstreams.  
That's not a realistic assumption IMHO.

> eg, if you want to use a kernel filesystem, you don't perform a check
> a kernel version number, because its inevitably wrong. You check
> /proc/filesystems to see if the filesystem is supported.
>    

And we have this today with QMP.  It provides a mechanism to query 
capabilities.  Fundamentally, this is a bootstrap problem and the best 
way to bootstrap is with a version number.

> > I think that's the sanest approach.  Just because a distro backports a
> > feature doesn't mean that it behaves like the upstream version.  libvirt
> > really needs to treat distro packages as separate entities from upstream
> > IMHO.
> >      
> If a distro changes the semantics of a upstream feature everyone looses
> no matter which way you look at it.
>    

It's a reality so we have to deal with it.

Regards,

Anthony Liguori

> Daniel
>