Using taskset , powertop and perf with qemu-kvm

[Jaspal <jaspal.iiith@gmail.com>]

Hello ,

I wish to run some cpu intensive tasks on a vm , use only one core 
specifically #0 of the 6 available ( Xeon 3670 ) and all the while 
record basic perf events like cpu-cycles , instructions etc.

I am using taskset to define the cpu-affinity of the qemu process :
taskset -p 0x1 `pgrep qemu` (assuming only 1 vm is running )

I can verify that the vm is now running only on core 0 using top. top 
also shows that the qemu process is using 99% of it's share of CPU time 
( %CPU column in top ) upon running the tasks in vm. However what I fail 
to see is the increase in time spent of the core 0 in the C0 ( active ) 
state via powertop. It barely manages to cross 12%. If I run the same 
tasks ( FFT benchmark ) on the host on a different core ( say #1 ) and 
monitor it using powertop again , I can see that  C0 % for core 1 is 
above 100% ( 104 ~ 107 )
So why does not the output of top ( qemu is using 99% of it's cpu time , 
which is spent on core 0 )  co-relate with powertop ( which says core 0 
is idle for around 88% of time ) ?
Also , powertop reports that core 2 is being used heavily ( 104 % ) and 
I suspect this is one which is executing the vm operations. But why ?
There are no other cpu/memory intensive tasks running on the host.

Next , I am using the perf kvm record function to save the stats. If I 
use all the 7 core counters available along-with -a option , perf 
generates ~2MB/second . Is there a way to save just the raw count of the 
events ?
Also , if instead of -a , if I give -C 0 , perf just saves around 
0.016MB/s as opposed to 1.894MB/s in case of -a option. If the vm is 
indeed using core 0 , then perf should have recorded the events.

On a last note , does perf kvm automatically selects the vm to profile ( 
in case multiple vms are running ) on the basis of the guest.kallsyms 
and guest.modules files passed to it ?

Thanks ,
Jaspal