Re: [PATCH 4/5] tsc: wire up entropy generation function

[Venkatesh Pallipadi <venki@xxxxxxxxxx>]

On Mon, Jun 13, 2011 at 3:06 PM, Jarod Wilson <jarod@xxxxxxxxxx> wrote:
> TSC is high enough resolution that we can use its low-order byte to
> stir new data into the random number generator entropy pool.

>From what I vaguely remember from years past, rdtsc, especially last
few bits of it are not very good as random number source. As they are
based on lower bus frequency and a multiplier. May be things have
changed these days. Adding Peter and Suresh for comments.

Thanks,
Venki

>
> CC: Matt Mackall <mpm@xxxxxxxxxxx>
> CC: "Venkatesh Pallipadi (Venki)" <venki@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxx>
> CC: John Stultz <johnstul@xxxxxxxxxx>
> CC: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> CC: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Signed-off-by: Jarod Wilson <jarod@xxxxxxxxxx>
> ---
>  arch/x86/kernel/tsc.c |   18 ++++++++++++++++++
>  1 files changed, 18 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 6cc6922..d206ec3 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -10,6 +10,7 @@
>  #include <linux/clocksource.h>
>  #include <linux/percpu.h>
>  #include <linux/timex.h>
> +#include <linux/random.h>
>
>  #include <asm/hpet.h>
>  #include <asm/timer.h>
> @@ -768,11 +769,28 @@ static void resume_tsc(struct clocksource *cs)
>        clocksource_tsc.cycle_last = 0;
>  }
>
> +static void tsc_add_entropy(void)
> +{
> +       static u64 last;
> +       u64 counter;
> +       int delta;
> +
> +       rdtscll(counter);
> +       delta = (int)(counter - last);
> +       last = counter;
> +
> +       if (delta == counter)
> +               return;
> +
> +       add_clocksource_randomness(delta);
> +}
> +
>  static struct clocksource clocksource_tsc = {
>        .name                   = "tsc",
>        .rating                 = 300,
>        .read                   = read_tsc,
>        .resume                 = resume_tsc,
> +       .entropy                = tsc_add_entropy,
>        .mask                   = CLOCKSOURCE_MASK(64),
>        .flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
>                                  CLOCK_SOURCE_MUST_VERIFY,
> --
> 1.7.1
>
>
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