On Wed, Jul 24, 2013 at 12:03:20PM -0400, Prarit Bhargava wrote:
>
> The TSC can have non-zero values at boot time on Intel Xeon E5 (family 6,
> model 45) aka "SandyBridge" processors. This is documented in the Errata
> for the E5 processors as BT81.
>
> The __cycles_2_ns() calculation is known to overflow if a large value of
> cycles is passed into the function. This is done by design to improve
> precision for smaller significant digits in the calculation. Since the E5
> processor can pass in a large value, we need to snapshot the TSC's
> initial value to avoid calculation overflows in the conversions of cycles
> to nanoseconds.
>
> Tested successfully on various Sandybridge systems as well as a few older
> and newer systems without any issues.
>
> Signed-off-by: Prarit Bhargava <pra...@redhat.com>
> Cc: John Stultz <john.stu...@linaro.org>
> Cc: Dave Hansen <d...@sr71.net>
> Cc: x...@kernel.org
> ---
> arch/x86/include/asm/timer.h | 15 +++------------
> arch/x86/kernel/tsc.c | 13 +++++++++++++
> 2 files changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
> index 34baa0e..f9d666b 100644
> --- a/arch/x86/include/asm/timer.h
> +++ b/arch/x86/include/asm/timer.h
> @@ -12,6 +12,8 @@ extern int recalibrate_cpu_khz(void);
>
> extern int no_timer_check;
>
> +extern unsigned long long tsc_initial_value;
> +
> /* Accelerators for sched_clock()
> * convert from cycles(64bits) => nanoseconds (64bits)
> * basic equation:
> @@ -59,21 +61,10 @@ static inline unsigned long long __cycles_2_ns(unsigned
> long long cyc)
> {
> int cpu = smp_processor_id();
> unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
> + cyc -= tsc_initial_value;
> ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
> (1UL << CYC2NS_SCALE_FACTOR));
> return ns;
> }
Hurm.. but eventually the TSC value _will_ get that large again, right?
So shouldn't we fix the actual problem?
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