title: why does perf stat show "stalled-cycles-backend" as <not supported>?
body: <p>Running <code>perf stat ls</code> shows this:</p>
  <pre><code>Performance counter stats for 'ls':
  
            1.388670 task-clock                #    0.067 CPUs utilized          
                   2 context-switches          #    0.001 M/sec                  
                   0 cpu-migrations            #    0.000 K/sec                  
                 266 page-faults               #    0.192 M/sec                  
             3515391 cycles                    #    2.531 GHz                    
             2096636 stalled-cycles-frontend   #   59.64% frontend cycles idle   
     &lt;not supported&gt; stalled-cycles-backend  
             2927468 instructions              #    0.83  insns per cycle        
                                               #    0.72  stalled cycles per insn
              615636 branches                  #  443.328 M/sec                  
               22172 branch-misses             #    3.60% of all branches        
  
         0.020657192 seconds time elapsed
  </code></pre>
  <p>Why is <em>stalled-cycles-backend</em> shown as &quot;not supported&quot;? What kind of CPU, hardware, kernel or user-space software do I need to see this value?</p>
  <p>Currently tried this on RHEL with Linux 3.12 for x86_64, with matching <code>perf</code> version, on different Intel Core i5 and i7 systems (Ivy Bridge type). None of them support <em>stalled-cycles-backend</em>.</p>
  <p>Some more info:</p>
  <pre><code>$ perf list | grep stalled
    stalled-cycles-frontend OR idle-cycles-frontend    [Hardware event]
    stalled-cycles-frontend OR cpu/stalled-cycles-frontend/ [Kernel PMU event]
  
  $ ls /sys/devices/cpu/events/
  branch-instructions  bus-cycles    cache-references  instructions  mem-stores
  branch-misses        cache-misses  cpu-cycles        mem-loads     stalled-cycles-frontend
  
  $ cat /sys/bus/event_source/devices/cpu/events/stalled-cycles-frontend
  event=0x0e,umask=0x01,inv,cmask=0x01
  </code></pre>
  <p><strong>Edit:</strong> just tried this on an AMD Phenom II X6 1045T CPU, under Ubuntu 12.04 with Linux 3.2 (32bit) - and here it does show values for both <em>stalled-cycles-frontend</em> and <em>stalled-cycles-backend</em>.</p>