On 30/11/16 19:17, Wei Huang wrote:
>
>
> 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@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...
It would have been good to report it earlier. Anyway, I've identified
the root issue, which seems to boil down to how you interpret a small
corner of the PMU architecture. I've raised it with the architecture
team here, and I should have a workaround/fix shortly.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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