On Mon, Jun 13, 2011 at 3:06 PM, Jarod Wilson <ja...@redhat.com> 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 <m...@selenic.com>
> CC: "Venkatesh Pallipadi (Venki)" <ve...@google.com>
> CC: Thomas Gleixner <t...@linutronix.de>
> CC: Ingo Molnar <mi...@elte.hu>
> CC: John Stultz <johns...@us.ibm.com>
> CC: Herbert Xu <herb...@gondor.apana.org.au>
> CC: "David S. Miller" <da...@davemloft.net>
> Signed-off-by: Jarod Wilson <ja...@redhat.com>
> ---
> 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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