Re: [PATCH] perf/x86/amd: check event before enable to avoid GPF

[Ravi Bangoria <ravi.bangoria@amd.com>]

On 6/8/2024 12:43 AM, George Kennedy wrote:
> Hi Ravi,
> 
> On 6/4/2024 9:40 AM, Ravi Bangoria wrote:
>>> On 6/4/2024 9:16 AM, Ravi Bangoria wrote:
>>>>>>>> Events can be deleted and the entry can be NULL.
>>>>>>> Can you please also explain "how".
>>>>>> It looks like x86_pmu_stop() is clearing the bit in active_mask and setting the events entry to NULL (and doing it in the correct order) for the same events index that amd_pmu_enable_all() is trying to enable.
>>>>>>>> Check event for NULL in amd_pmu_enable_all() before enable to avoid a GPF.
>>>>>>>> This appears to be an AMD only issue.
>>>>>>>>
>>>>>>>> Syzkaller reported a GPF in amd_pmu_enable_all.
>>>>>>> Can you please provide a bug report link? Also, any reproducer?
>>>>>> The Syzkaller reproducer can be found in this link:
>>>>>> https://lore.kernel.org/netdev/CAMt6jhyec7-TSFpr3F+_ikjpu39WV3jnCBBGwpzpBrPx55w20g@mail.gmail.com/T/#u
>>>>>>>> @@ -760,7 +760,8 @@ static void amd_pmu_enable_all(int added)
>>>>>>>>             if (!test_bit(idx, cpuc->active_mask))
>>>>>>>>                 continue;
>>>>>>>>     -        amd_pmu_enable_event(cpuc->events[idx]);
>>>>>>>> +        if (cpuc->events[idx])
>>>>>>>> +            amd_pmu_enable_event(cpuc->events[idx]);
>>>>>>> What if cpuc->events[idx] becomes NULL after if (cpuc->events[idx]) but
>>>>>>> before amd_pmu_enable_event(cpuc->events[idx])?
>>>>>> Good question, but the crash has not reproduced with the proposed fix in hours of testing. It usually reproduces within minutes without the fix.
>>>>> Also, a similar fix is done in __intel_pmu_enable_all() in arch/x86/events/intel/core.c except that a WARN_ON_ONCE is done as well.
>>>>> See: https://elixir.bootlin.com/linux/v6.10-rc1/source/arch/x86/events/intel/core.c#L2256
>>>> There are subtle differences between Intel and AMD pmu implementation.
>>>> __intel_pmu_enable_all() enables all event with single WRMSR whereas
>>>> amd_pmu_enable_all() loops over each PMC and enables it individually.
>>>>
>>>> The WARN_ON_ONCE() is important because it will warn about potential
>>>> sw bug somewhere else.
>>> We could add a similar WARN_ON_ONCE() to the proposed patch.
>> Sure, that would help in future. But for current splat, can you please
>> try to rootcause the underlying race condition?
> 
> Were you able to reproduce the crash on the AMD machine?

I'm able to reproduce within the KVM guest. Will try to investigate further.

Thanks,
Ravi