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

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Message-ID: <4F3D0CB1.5070707@redhat.com>
Date:	Thu, 16 Feb 2012 16:03:29 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Igor Mammedov <imammedo@...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 <amit.shah@...hat.com>,
	mtosatti@...hat.com
Subject: Re: [PATCH] BUG in pv_clock when overflow condition is detected

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.


-- 
error compiling committee.c: too many arguments to function

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