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random(4): depessimize arc4random

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- __predict_false reseeding on entry as it is almost never true.
- don't blindly atomic_cmpset as on x86 it ends up dirtying the cacheline.
it almost ever succeeds per above
- fetch the timestamp prior to getting the cpu number

Reviewed by:	cem
Approved by:	secteam (delphij)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20242
This commit is contained in:
mjg 2019-05-12 06:32:46 +00:00
parent 947c947081
commit 95034c44e6
1 changed files with 5 additions and 3 deletions
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8
sys/libkern/arc4random.c
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@ -173,18 +173,20 @@ arc4rand(void *ptr, u_int len, int reseed)
u_int length;
u_int8_t *p;
if (reseed || atomic_cmpset_int(&arc4rand_iniseed_state, ARC4_ENTR_HAVE, ARC4_ENTR_SEED))
if (__predict_false(reseed ||
(arc4rand_iniseed_state == ARC4_ENTR_HAVE &&
atomic_cmpset_int(&arc4rand_iniseed_state, ARC4_ENTR_HAVE, ARC4_ENTR_SEED))))
CHACHA20_FOREACH(chacha20)
chacha20_randomstir(chacha20);
chacha20 = &chacha20inst[curcpu];
getmicrouptime(&tv);
chacha20 = &chacha20inst[curcpu];
/* We may get unlucky and be migrated off this CPU, but that is expected to be infrequent */
if ((chacha20->numbytes > CHACHA20_RESEED_BYTES) || (tv.tv_sec > chacha20->t_reseed))
chacha20_randomstir(chacha20);
mtx_lock(&chacha20->mtx);
p = ptr;
mtx_lock(&chacha20->mtx);
while (len) {
length = MIN(CHACHA20_BUFFER_SIZE, len);
chacha_encrypt_bytes(&chacha20->ctx, chacha20->m_buffer, p, length);