Re: [Patch] Fix the possibility of insane return value of hpet_calibrate()against SMI. (take 2)

[Paul Gortmaker]

Clemens Ladisch wrote:
> Paul Gortmaker wrote:
>   
> > Yasunori Goto wrote:
> >     
> > > +	/*
> > > +	 * Try to calibrate until return value becomes stable small value.
> > > +	 * If SMI interruption occurs in calibration loop, the return value
> > > +	 * will be big. This avoids its impact.
> > > +	 */
> > > +	do {
> > > +		tmp = __hpet_calibrate(hpetp);
> > > +		if (ret <= tmp)
> > > +			break;
> > > +		ret = tmp;
> > > +	} while (1);
> > >       
> > For what it is worth, if a situation arose where continued calls to
> > hpet_calibrate() represented a monotonically decreasing function,
> > (perhaps some insane power management?) you could be stuck
> > in this function for an unbounded amount of time.
> >     
>
> On my machine, the number of iterations of the loop in __hpet_calibarate
> (the value of i) is about 400, and since HPET reads/writes go to the
> southbridge, it cannot get much higher than about 1000 even on faster
> machines.
>
> The value of (m - start) is approximately constant, as long as the loop
> runs for about 1 ms, so there cannot be too many distinct return values.
>
> Consequently, the only way for perverse SMIs to produce more
> monotonically decreasing calibration values is to introduce delays that
> make the loop run longer than 1 ms, i.e., to eat most of the CPU time
> for at least several seconds (or longer if you want unbounded time).
> Even in the real world ;-), no laptop manufacturer is that insane.
>   

Right - as I said, I didn't expect that it could ever happen, and your 
analysis
above seems to help reinforce that, so we probably should be OK.

Thanks,
Paul.

> Best regards,
> Clemens
>   

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