Implement cpu_rnd_messybits() as a read of the virtual counter xored

with a bit-reversed copy of itself.  There is progressively less
entropy in the higher bits of a counter than in the lower bits, so
bit-reverse one half in order to extract maximal entropy.

style fixes and ok kettenis@

2 files changed, 12 insertions, 12 deletions

diff --git a/sys/arch/arm64/arm64/machdep.c b/sys/arch/arm64/arm64/machdep.c
index 4e292479233..db37ab929f3 100644
--- a/sys/arch/arm64/arm64/machdep.c
+++ b/sys/arch/arm64/arm64/machdep.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: machdep.c,v 1.52 2020/05/31 06:23:57 dlg Exp $ */
+/* $OpenBSD: machdep.c,v 1.53 2020/06/05 23:16:24 naddy Exp $ */
 /*
  * Copyright (c) 2014 Patrick Wildt <patrick@blueri.se>
  *
@@ -1247,12 +1247,3 @@ dumpregs(struct trapframe *frame)
 	printf("pc: 0x%016lx\n", frame->tf_elr);
 	printf("spsr: 0x%016lx\n", frame->tf_spsr);
 }
-
-unsigned int
-cpu_rnd_messybits(void)
-{
-	struct timespec ts;
-
-	nanotime(&ts);
-	return (ts.tv_nsec ^ (ts.tv_sec << 20));
-}
diff --git a/sys/arch/arm64/include/cpu.h b/sys/arch/arm64/include/cpu.h
index 099567d225c..a4d18502997 100644
--- a/sys/arch/arm64/include/cpu.h
+++ b/sys/arch/arm64/include/cpu.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: cpu.h,v 1.17 2020/05/31 06:23:57 dlg Exp $ */
+/* $OpenBSD: cpu.h,v 1.18 2020/06/05 23:16:24 naddy Exp $ */
 /*
  * Copyright (c) 2016 Dale Rahn <drahn@dalerahn.com>
  *
@@ -183,7 +183,16 @@ void cpu_boot_secondary_processors(void);
 
 #define curpcb		curcpu()->ci_curpcb
 
-unsigned int		cpu_rnd_messybits(void);
+static inline unsigned int
+cpu_rnd_messybits(void)
+{
+	uint64_t val, rval;
+
+	__asm volatile("mrs %0, CNTVCT_EL0; rbit %1, %0;"
+	    : "=r" (val), "=r" (rval));
+
+	return (val ^ rval);
+}
 
 /*
  * Scheduling glue