linux-kernel - Re: [Patch] Fix the possibility of insane return value of hpet

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Date:	Mon, 16 Mar 2009 11:34:55 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	clemens@...isch.de, Linux Kernel ML <linux-kernel@...r.kernel.org>,
	robert.picco@...com, venkatesh.pallipadi@...el.com,
	vojtech@...e.cz, mingo@...hat.com
Subject: Re: [Patch] Fix the possibility of insane return value of  hpet_calibrate() against SMI.

> On Fri, Mar 13, 2009 at 1:00 AM, Yasunori Goto <y-goto@...fujitsu.com> wrote:
> > Hello.
> >
> > I think there is a possibility that HPET driver will return
> > insane value due to a SMI interruption (or switching guests by hypervisor).
> > I found it by reviewing, and I would like to fix it.
> >
> > Current HPET driver calibrates the adjustment value
> > by calculation the elapse time in CPU busy loop.
> > However this way is too dangerous against SMI interruption.
> >
> > Here is the calibration code in hpet_calibrate()
> >
> > ?701 static unsigned long hpet_calibrate(struct hpets *hpetp)
> > ? ? ? ? ? ? :
> > ? ? ? ? ? ? :
> > ?728 ? ? ? ? do {
> > ?729 ? ? ? ? ? ? ? ? m = read_counter(&hpet->hpet_mc);
> > ?730 ? ? ? ? ? ? ? ? write_counter(t + m + hpetp->hp_delta, &timer->hpet_compare);
> > ?731 ? ? ? ? } while (i++, (m - start) < count);
> > ?732
> > ?733 ? ? ? ? local_irq_restore(flags);
> > ?734
> > ?735 ? ? ? ? return (m - start) / i;
> >
> > If SMI interruption occurs between 728 to 731, then return value will be
> > bigger value than correct one. (SMI is not able to be controlled by OS.)
> >
> >
> > This patch is a simple solution to fix it.
> > hpet_calibrate() is called 5 times, and one of them is expected as
> > correct value.
> 
> I've no sense of feel for how long each calibration run would take.
> Would doing it 5 times show up as a significant increase in the boot
> time for those that care about boot time being as quick as possible?

Hmm. The loop times is trade off against reliable value....
Though SMI is rare interruption, I don't know how frequent
hypervisor's switch is.

Each calibration of this has 1 milli second.
Do you think 5 msec is too long?
If yes, how is 3 msec? Is it still too long?

Thanks.

-- 
Yasunori Goto 


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