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Subject[PATCH, RFC 11/12] random: speed up the fast_mix function by a factor of four
Date
By mixing the entropy in chunks of 32-bit words instead of byte by
byte, we can speed up the fast_mix function significantly. Since it
is called on every single interrupt, on systems with a very heavy
interrupt load, this can make a noticeable different.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Jörn Engel <joern@logfs.org>
---
drivers/char/random.c | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 3439b1c..86879d1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -583,21 +583,26 @@ struct fast_pool {
* collector. It's hardcoded for an 128 bit pool and assumes that any
* locks that might be needed are taken by the caller.
*/
-static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
+static void fast_mix(struct fast_pool *f, __u32 input[4])
{
- const char *bytes = in;
__u32 w;
- unsigned i = f->count;
unsigned input_rotate = f->rotate;

- while (nbytes--) {
- w = rol32(*bytes++, input_rotate & 31) ^ f->pool[i & 3] ^
- f->pool[(i + 1) & 3];
- f->pool[i & 3] = (w >> 3) ^ twist_table[w & 7];
- input_rotate += (i++ & 3) ? 7 : 14;
- }
- f->count = i;
+ w = rol32(input[0], input_rotate) ^ f->pool[0] ^ f->pool[3];
+ f->pool[0] = (w >> 3) ^ twist_table[w & 7];
+ input_rotate = (input_rotate + 14) & 31;
+ w = rol32(input[1], input_rotate) ^ f->pool[1] ^ f->pool[0];
+ f->pool[1] = (w >> 3) ^ twist_table[w & 7];
+ input_rotate = (input_rotate + 7) & 31;
+ w = rol32(input[2], input_rotate) ^ f->pool[2] ^ f->pool[1];
+ f->pool[2] = (w >> 3) ^ twist_table[w & 7];
+ input_rotate = (input_rotate + 7) & 31;
+ w = rol32(input[3], input_rotate) ^ f->pool[3] ^ f->pool[2];
+ f->pool[3] = (w >> 3) ^ twist_table[w & 7];
+ input_rotate = (input_rotate + 7) & 31;
+
f->rotate = input_rotate;
+ f->count++;
}

/*
@@ -836,10 +841,9 @@ void add_interrupt_randomness(int irq, int irq_flags)
input[3] = ip >> 32;
}

- fast_mix(fast_pool, input, sizeof(input));
+ fast_mix(fast_pool, input);

- if ((fast_pool->count & 1023) &&
- !time_after(now, fast_pool->last + HZ))
+ if ((fast_pool->count & 63) && !time_after(now, fast_pool->last + HZ))
return;

fast_pool->last = now;
--
1.7.12.rc0.22.gcdd159b
--
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