Re: Using get_cycles for add_timer_randomness

[Anton Blanchard <anton@samba.org>]

This sounded like a good idea at the time, any reason we cant merge
this?

Anton

On Sat, Aug 16, 2003 at 03:03:17AM +0200, Arnd Bergmann wrote:
> I noticed that only i386 and x86-64 are currently using
> a high resolution timer source when adding randomness.
> Since many architectures have a working get_cycles()
> implementation, it seems rather straightforward to use
> that.
> 
> Has this been discussed before, or can anyone comment
> on the implementation below?
> 
> This patch attempts to take into account the size of
> cycles_t, which is either 32 or 64 bits wide but
> independent of the architecture's word size.
> 
> The behavior should be nearly identical to the
> old one on i386, x86-64 and all architectures
> without a time stamp counter, while finding
> more entropy on the other architectures.
> 
> 	Arnd <><
> 
> ===== drivers/char/random.c 1.35 vs edited =====
> --- 1.35/drivers/char/random.c	Wed Aug  6 19:59:31 2003
> +++ edited/drivers/char/random.c	Sat Aug 16 02:05:34 2003
> @@ -711,8 +711,8 @@
>  
>  /* There is one of these per entropy source */
>  struct timer_rand_state {
> -	__u32		last_time;
> -	__s32		last_delta,last_delta2;
> +	cycles_t	last_time;
> +	long		last_delta,last_delta2;
>  	int		dont_count_entropy:1;
>  };
>  
> @@ -729,27 +729,28 @@
>   * The number "num" is also added to the pool - it should somehow describe
>   * the type of event which just happened.  This is currently 0-255 for
>   * keyboard scan codes, and 256 upwards for interrupts.
> - * On the i386, this is assumed to be at most 16 bits, and the high bits
> - * are used for a high-resolution timer.
> + * This is assumed to be at most 16 bits, and the high bits are used for
> + * high-resolution timers.
>   *
>   */
>  static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
>  {
> -	__u32		time;
> -	__s32		delta, delta2, delta3;
> +	cycles_t	time;
> +	long		delta, delta2, delta3;
>  	int		entropy = 0;
>  
> -#if defined (__i386__) || defined (__x86_64__)
> -	if (cpu_has_tsc) {
> -		__u32 high;
> -		rdtsc(time, high);
> -		num ^= high;
> +	/*
> +	 * Use get_cycles() if implemented, otherwise fall back to
> +	 * jiffies.
> +	 */
> +	time = get_cycles();
> +	if (time != 0) {
> +		if (sizeof (time) > 4) {
> +			num ^= (u32)(time >> 32);
> +		}
>  	} else {
>  		time = jiffies;
>  	}
> -#else
> -	time = jiffies;
> -#endif
>  
>  	/*
>  	 * Calculate number of bits of randomness we probably added.
> ===== include/asm-i386/timex.h 1.5 vs edited =====
> --- 1.5/include/asm-i386/timex.h	Mon Jun  9 14:41:23 2003
> +++ edited/include/asm-i386/timex.h	Sat Aug 16 02:17:05 2003
> @@ -7,7 +7,7 @@
>  #define _ASMi386_TIMEX_H
>  
>  #include <linux/config.h>
> -#include <asm/msr.h>
> +#include <asm/processor.h>
>  
>  #ifdef CONFIG_X86_PC9800
>     extern int CLOCK_TICK_RATE;
> @@ -44,14 +44,17 @@
>  
>  static inline cycles_t get_cycles (void)
>  {
> +	unsigned long long ret=0;
> +
>  #ifndef CONFIG_X86_TSC
> -	return 0;
> -#else
> -	unsigned long long ret;
> +	if (!cpu_has_tsc)
> +		return 0;
> +#endif
>  
> +#if defined(CONFIG_X86_GENERIC) || defined(CONFIG_X86_TSC)
>  	rdtscll(ret);
> -	return ret;
>  #endif
> +	return ret;
>  }
>  
>  extern unsigned long cpu_khz;
> -
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