Re: [RFC patch 0/4] TSC calibration improvements

[Ingo Molnar]

> +static unsigned long quick_pit_calibrate(void)
> +{
[...]
> +	if (pit_expect_msb(0xff)) {
> +		int i;
> +		u64 t1, t2, delta;
> +		unsigned char expect = 0xfe;
> +
> +		t1 = get_cycles();
> +		for (i = 0; i < QUICK_PIT_ITERATIONS; i++, expect--) {
> +			if (!pit_expect_msb(expect))
> +				goto failed;
> +		}
> +		t2 = get_cycles();

hm, unless i'm missing something i think here we still have a small 
window for an SMI or some virtualization delay to slip in and cause 
massive inaccuracy: if the delay happens _after_ the last 
pit_expect_msb() and _before_ the external get_cycles() call. Right?

i fixed that by adding one more pit_expect_msb() call.

plus i think QUICK_PIT_ITERATIONS is quite close to overflowing 255 
which is built into the u32 'expect' variable (the MSB will only 
overflow to 10 bits or so) - so i've added a BUILD_BUG_ON() to make sure 
anyone tuning QUICK_PIT_MS above 60msec or so would get a build error 
instead of some hard(er) to track down calibration error.

but it's getting late here so please double-check me ... The commit is 
below.

	Ingo

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