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SubjectRe: [PATCH] BUG in pv_clock when overflow condition is detected
On 02/16/2012 03:03 PM, Avi Kivity wrote:
> On 02/15/2012 07:18 PM, Igor Mammedov wrote:
>>> On 02/15/2012 01:23 PM, Igor Mammedov wrote:
>>>>>> static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time
>>>>>> *shadow)
>>>>>> {
>>>>>> - u64 delta = native_read_tsc() - shadow->tsc_timestamp;
>>>>>> + u64 delta;
>>>>>> + u64 tsc = native_read_tsc();
>>>>>> + BUG_ON(tsc< shadow->tsc_timestamp);
>>>>>> + delta = tsc - shadow->tsc_timestamp;
>>>>>> return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
>>>>>> shadow->tsc_shift);
>>>>>
>>>>> Maybe a WARN_ON_ONCE()? Otherwise a relatively minor hypervisor
>>>>> bug can
>>>>> kill the guest.
>>>>
>>>>
>>>> An attempt to print from this place is not perfect since it often
>>>> leads
>>>> to recursive calling to this very function and it hang there
>>>> anyway.
>>>> But if you insist I'll re-post it with WARN_ON_ONCE,
>>>> It won't make much difference because guest will hang/stall due
>>>> overflow
>>>> anyway.
>>>
>>> Won't a BUG_ON() also result in a printk?
>> Yes, it will. But stack will still keep failure point and poking
>> with crash/gdb at core will always show where it's BUGged.
>>
>> In case it manages to print dump somehow (saw it couple times from ~
>> 30 test cycles), logs from console or from kernel message buffer
>> (again poking with gdb) will show where it was called from.
>>
>> If WARN* is used, it will still totaly screwup clock and
>> "last value" and system will become unusable, requiring looking with
>> gdb/crash at the core any way.
>>
>> So I've just used more stable failure point that will leave trace
>> everywhere it manages (maybe in console log, but for sure in stack)
>> in case of WARN it might leave trace on console or not and probably
>> won't reflect failure point in stack either leaving only kernel
>> message buffer for clue.
>>
>
> Makes sense. But do get an ack from the Xen people to ensure this
> doesn't break for them.
>
Konrad, Ian

Could you please review patch form point of view of xen?
Whole thread could be found here https://lkml.org/lkml/2012/2/13/286

--
Thanks,
Igor


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