Unable to use perf in VM

[Wei Huang]

On 11/30/2016 07:37 AM, Marc Zyngier wrote:
> On 30/11/16 11:48, Marc Zyngier wrote:
>> + 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).
> 
> It turns out that we *don't* inject an undef. It seems to be generated
> locally at EL1.
> 
> Still digging.

Just FYI: I saw it on Mustang before. My initial thought was HW related,
but without proof. I am interested to see your findings...

> 
> 	M.
>