Unable to use perf in VM

[marc.zyngier@arm.com (Marc Zyngier)]

+ Shannon

On 29/11/16 22:04, Itaru Kitayama wrote:
> Hi,
> 
> In a VM (virsh controlled, KVM acceleration enabled) on a recent
> kvmarm kernel host, I find I am unable to use perf to obtain
> performance statistics for a complex task like kernel build.
> (I've verified this is seen with a Fedora 25 VM and host combination
> as well)
> APM folks CC'ed think this might be caused by a bug in the core PMU 
> framework code, thus I'd like to have experts opinion on this issue.
> 
> [root at localhost linux]# perf stat -B make
>    CHK     include/config/kernel.release
> [  119.617684] git[1144]: undefined instruction: pc=fffffc000808ff30
> [  119.623040] Code: 51000442 92401042 d51b9ca2 d5033fdf (d53b9d40)
> [  119.627607] Internal error: undefined instruction: 0 [#1] SMP

[...]

In a VM running mainline hosted on an AMD Seattle box:

 Performance counter stats for 'make':

    1526089.499304      task-clock:u (msec)       #    0.932 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
          29527793      page-faults:u             #    0.019 M/sec                  
     2913174122673      cycles:u                  #    1.909 GHz                    
     2365040892322      instructions:u            #    0.81  insn per cycle         
   <not supported>      branches:u                                                  
       32049215378      branch-misses:u           #    0.00% of all branches        

    1637.531444837 seconds time elapsed

Running the same host kernel on a Mustang system, the guest explodes
in the way you reported. The failing instruction always seems to be
an access to pmxevcntr_el0 (I've seen both reads and writes).

Funnily enough, it dies if you try any HW event other than cycles
("perf stat -e cycles ls" works, and "perf stat -e instructions ls"
explodes). Which would tend to indicate that we're screwing up
the counter selection, but I have no proof of that (specially that
the Seattle guest is working just as expected).

Shannon, any idea?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...