Date: Sat, 3 Aug 2013 10:24:40 +0100 From: Mark R V Murray <mark@grondar.org> To: "freebsd-arm@freebsd.org" <arm@freebsd.org> Subject: PATCH: get_cyclecount() on ARMv6 and better Message-ID: <78D22A66-86E5-43B1-ABCA-7BF14F8061AB@grondar.org>
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Hi folks
The CSPRNG used to drive /dev/random is Yarrow, and it needs good timing =
jitter to produce decent numbers.
The i86_32 and i86_64 platforms both have the TSC register, wrapped in =
the get_cyclecount() inline function, but the ARM platform doesn't use =
its equivalent, the CCNT register. The alternative, using system time, =
loses LOTS of low-bit jitter, and is certainly worth improving upon.
I'm at the early stages of testing the patch below (I only have RPi), =
and would like to get some comments and reviews, please. I am very =
doubtful indeed that I got the #ifdefs right - they are a bit of a =
minefield! ;-)
The patch returns the 32-bit CCNT register as the 64-bit quantity that =
get_cyclecount() provides on all platforms; this is a very minor =
problem, but I suppose I could figure out some kind of crude carry =
mechanism and force the number to increment beyond 32 bits; I doubt its =
worth it though, as its the low bits that provide the jitter.
Thanks in advance!
M
--=20
Mark R V Murray
Index: cpu.h
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
--- cpu.h (revision 253832)
+++ cpu.h (working copy)
@@ -5,6 +5,9 @@
#define MACHINE_CPU_H
=20
#include <machine/armreg.h>
+#ifndef _KERNEL
+#include <machine/sysarch.h>
+#endif
=20
void cpu_halt(void);
void swi_vm(void *);
@@ -13,11 +16,26 @@
static __inline uint64_t
get_cyclecount(void)
{
+#if defined (__ARM_ARCH_7__) || \
+ defined (__ARM_ARCH_7A__) || \
+ defined (__ARM_ARCH_6__) || \
+ defined (__ARM_ARCH_6J__) || \
+ defined (__ARM_ARCH_6K__) || \
+ defined (__ARM_ARCH_6T2__) || \
+ defined (__ARM_ARCH_6Z__) || \
+ defined (__ARM_ARCH_6ZK__)
+
+ uint32_t ccnt;
+
+ /* Read CCNT. Darn; its only 32 bits. */
+ __asm __volatile("mrc p15, 0, %0, c9, c13, 0": "=3Dr" (ccnt));
+ return ((uint64_t)ccnt);
+#else
struct bintime bt;
=20
binuptime(&bt);
return ((uint64_t)bt.sec << 56 | bt.frac >> 8);
- =09
+#endif
}
#endif
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