linux-kernel - Re: [PATCH] BUG in pv_clock when overflow condition is detected

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Date:	Wed, 15 Feb 2012 12:23:17 +0100
From:	Igor Mammedov <imammedo@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	riel@...hat.com, amit.shah@...hat.com, mtosatti@...hat.com
Subject: Re: [PATCH] BUG in pv_clock when overflow condition is detected

On 02/15/2012 11:49 AM, Avi Kivity wrote:
> On 02/13/2012 08:20 PM, Igor Mammedov wrote:
>> BUG when overflow occurs at pvclock.c:pvclock_get_nsec_offset
>>
>>      u64 delta = native_read_tsc() - shadow->tsc_timestamp;
>>
>> this might happen at an attempt to read an uninitialized yet clock.
>> It won't prevent stalls and hangs but at least it won't do it silently.
>>
>> Signed-off-by: Igor Mammedov<imammedo@...hat.com>
>> ---
>>   arch/x86/kernel/pvclock.c |    5 ++++-
>>   1 files changed, 4 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
>> index 42eb330..35a6190 100644
>> --- a/arch/x86/kernel/pvclock.c
>> +++ b/arch/x86/kernel/pvclock.c
>> @@ -43,7 +43,10 @@ void pvclock_set_flags(u8 flags)
>>
>>   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.

If there is an intention to keep guest functional after the event then
maybe this patch is a way to go
   http://www.spinics.net/lists/kvm/msg68463.html
this way clock will be re-silent to this kind of errors, like bare-metal
one is.

-- 
Thanks,
  Igor
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