Re: [Xen-devel] [PATCH] BUG in pv_clock when overflow condition is detected

[Igor Mammedov <imammedo@xxxxxxxxxx>]

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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