Am 27.06.2010 um 10:34 schrieb Avi Kivity [off-list ref]:

On 06/26/2010 02:25 AM, Alexander Graf wrote:

quoted

We just introduced a new PV interface that screams for  
documentation. So here
it is - a shiny new and awesome text file describing the internal  
works of
the PPC KVM paravirtual interface.


+Querying for existence
+======================
+
+To find out if we're running on KVM or not, we overlay the PVR  
register. Usually
+the PVR register contains an id that identifies your CPU type. If,  
however, you
+pass KVM_PVR_PARA in the register that you want the PVR result in,  
the register
+still contains KVM_PVR_PARA after the mfpvr call.
+
+    LOAD_REG_IMM(r5, KVM_PVR_PARA)
+    mfpvr    r5
+    [r5 still contains KVM_PVR_PARA]
+
+Once determined to run under a PV capable KVM, you can now use  
hypercalls as
+described below.

On x86 we allow host userspace to determine whether the guest sees  
the paravirt interface (and what features are exposed).  This allows  
you to live migrate from a newer host to an older host, by not  
exposing the newer features.

A very good idea indeed. Let's postpone that to when we expose enough  
state to make live migration possible.

Alex