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

> +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

------------>
From 40d2650256289d3ba59c4fd146b86b972db6ec40 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@elte.hu>
Date: Thu, 4 Sep 2008 22:47:47 +0200
Subject: [PATCH] x86: quick TSC calibration, improve
- make sure the final TSC timestamp is reliable too

- make sure nobody increases QUICK_PIT_MS so that
QUICK_PIT_ITERATIONS can get larger than 0xff, breaking the iteration.
(It would take about 60 msecs to reach that limit.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/x86/kernel/tsc.c | 11 +++++++++++
1 files changed, 11 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 839070b..4832a40 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -304,6 +304,11 @@ static unsigned long quick_pit_calibrate(void)
outb(0xff, 0x42);
outb(0xff, 0x42);

+ /*
+ * The iteration assumes that expect never goes below zero:
+ */
+ BUILD_BUG_ON(QUICK_PIT_ITERATIONS >= 0xff);
+
if (pit_expect_msb(0xff)) {
int i;
u64 t1, t2, delta;
@@ -317,6 +322,12 @@ static unsigned long quick_pit_calibrate(void)
t2 = get_cycles();

/*
+ * Make sure we can rely on the second TSC timestamp:
+ */
+ if (!pit_expect_msb(--expect))
+ goto failed;
+
+ /*
* Ok, if we get here, then we've seen the
* MSB of the PIT decrement QUICK_PIT_ITERATIONS
* times, and each MSB had many hits, so we never

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