x86/tsc: Prevent 32bit truncation in calc_hpet_ref()
authorXiaoming Gao <gxm.linux.kernel@gmail.com>
Fri, 13 Apr 2018 09:48:08 +0000 (17:48 +0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 26 Apr 2018 09:02:02 +0000 (11:02 +0200)
commit08641a24d4e70607f767b066def49d3b3a3f9c98
tree41df8190b893536caa9413b98acbc11f73255072
parentc6aaaaa4d62ad885a8ca0a255d4af975f843ee98
x86/tsc: Prevent 32bit truncation in calc_hpet_ref()

commit d3878e164dcd3925a237a20e879432400e369172 upstream.

The TSC calibration code uses HPET as reference. The conversion normalizes
the delta of two HPET timestamps:

    hpetref = ((tshpet1 - tshpet2) * HPET_PERIOD) / 1e6

and then divides the normalized delta of the corresponding TSC timestamps
by the result to calulate the TSC frequency.

    tscfreq = ((tstsc1 - tstsc2 ) * 1e6) / hpetref

This uses do_div() which takes an u32 as the divisor, which worked so far
because the HPET frequency was low enough that 'hpetref' never exceeded
32bit.

On Skylake machines the HPET frequency increased so 'hpetref' can exceed
32bit. do_div() truncates the divisor, which causes the calibration to
fail.

Use div64_u64() to avoid the problem.

[ tglx: Fixes whitespace mangled patch and rewrote changelog ]

Signed-off-by: Xiaoming Gao <newtongao@tencent.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: peterz@infradead.org
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/38894564-4fc9-b8ec-353f-de702839e44e@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
arch/x86/kernel/tsc.c