Adrian Bunk wrote:

These are Linux specific operations.

Without an _GPL you are in the grey area where courts have to decide 
whether a module using this would be a derived work according to 
copyright law in $country_of_the_court and therefore has to be GPL.

With the _GPL, everything is clear without any lawyers involved.
  

Hardly.  The _GPL is a hint as to the intent of the author, but it is no 
more than a hint.

My intent here (and I think the intent of the other authors) is not to 
cause breakage of things which currently work, so the _GPL is not 
appropriate for that reason.  Paravirt_ops is a restatement of many 
interfaces which already exist in Linux in a non-_GPL form, so making 
the structure _GPL is effectively relicensing them.

    J