x86/tsc: Prevent 32bit truncation in calc_hpet_ref()

[ Upstream commit d3878e164dcd3925a237a20e879432400e369172 ]

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: Sasha Levin <alexander.levin@microsoft.com>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 37bae0d677143a1a27f23607c676f941cbd29a80..8fdcdbf5f30957950ac30f45956893522b4af374 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -399,7 +399,7 @@ static unsigned long calc_hpet_ref(u64 deltatsc, u64 hpet1, u64 hpet2)
        hpet2 -= hpet1;
        tmp = ((u64)hpet2 * hpet_readl(HPET_PERIOD));
        do_div(tmp, 1000000);
-       do_div(deltatsc, tmp);
+       deltatsc = div64_u64(deltatsc, tmp);
 
        return (unsigned long) deltatsc;
 }