Often when debugging performance problems in a virtualized environment 
you need to correlate what is happening in the guest with what is 
happening in the host. To correlate events you need a common time basis 
(or the ability to directly correlate the two).

The attached patch paravirtualizes perf_clock, pulling the timestamps in 
VMs from the host using an MSR read if the option is available (exposed 
via KVM feature flag). I realize this is not the correct end code but it 
illustrates what I would like to see -- host and guests using the same 
perf_clock so timestamps directly correlate.

Any suggestions on how to do this and without impacting performance. I 
noticed the MSR path seems to take about twice as long as the current 
implementation (which I believe results in rdtsc in the VM for x86 with 
stable TSC).

David