From b6e61eef4f9f94714ac3ee4a5c96862d9bcd1836 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 31 Jul 2009 12:45:41 -0700 Subject: [PATCH] x86: Fix serialization in pit_expect_msb() Wei Chong Tan reported a fast-PIT-calibration corner-case: | pit_expect_msb() is vulnerable to SMI disturbance corner case | in some platforms which causes /proc/cpuinfo to show wrong | CPU MHz value when quick_pit_calibrate() jumps to success | section. I think that the real issue isn't even an SMI - but the fact that in the very last iteration of the loop, there's no serializing instruction _after_ the last 'rdtsc'. So even in the absense of SMI's, we do have a situation where the cycle counter was read without proper serialization. The last check should be done outside the outer loop, since _inside_ the outer loop, we'll be testing that the PIT has the right MSB value has the right value in the next iteration. So only the _last_ iteration is special, because that's the one that will not check the PIT MSB value any more, and because the final 'get_cycles()' isn't serialized. In other words: - I'd like to move the PIT MSB check to after the last iteration, rather than in every iteration - I think we should comment on the fact that it's also a serializing instruction and so 'fences in' the TSC read. Here's a suggested replacement. Signed-off-by: Linus Torvalds Reported-by: "Tan, Wei Chong" Tested-by: "Tan, Wei Chong" LKML-Reference: Signed-off-by: Ingo Molnar --- arch/x86/kernel/tsc.c | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-) diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 6e1a368d21d4..71f4368b357e 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -275,15 +275,20 @@ static unsigned long pit_calibrate_tsc(u32 latch, unsigned long ms, int loopmin) * use the TSC value at the transitions to calculate a pretty * good value for the TSC frequencty. */ +static inline int pit_verify_msb(unsigned char val) +{ + /* Ignore LSB */ + inb(0x42); + return inb(0x42) == val; +} + static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap) { int count; u64 tsc = 0; for (count = 0; count < 50000; count++) { - /* Ignore LSB */ - inb(0x42); - if (inb(0x42) != val) + if (!pit_verify_msb(val)) break; tsc = get_cycles(); } @@ -336,8 +341,7 @@ static unsigned long quick_pit_calibrate(void) * to do that is to just read back the 16-bit counter * once from the PIT. */ - inb(0x42); - inb(0x42); + pit_verify_msb(0); if (pit_expect_msb(0xff, &tsc, &d1)) { for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) { @@ -348,8 +352,19 @@ static unsigned long quick_pit_calibrate(void) * Iterate until the error is less than 500 ppm */ delta -= tsc; - if (d1+d2 < delta >> 11) - goto success; + if (d1+d2 >= delta >> 11) + continue; + + /* + * Check the PIT one more time to verify that + * all TSC reads were stable wrt the PIT. + * + * This also guarantees serialization of the + * last cycle read ('d2') in pit_expect_msb. + */ + if (!pit_verify_msb(0xfe - i)) + break; + goto success; } } printk("Fast TSC calibration failed\n");