[PATCH] Using get_cycles for add_timer_randomness

I tested how long it took to do a dd from /dev/random on ppc64 before and
after this patch, while doing a ping flood from another machine.

before:
# /usr/bin/time dd if=/dev/random of=/dev/zero count=1k
0+51 records in
Command terminated by signal 2
0.00user 0.00system 19:18.46elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k

I gave up after 19 minutes.

after:
# /usr/bin/time dd if=/dev/random of=/dev/zero count=1k
0+1024 records in
0.00user 0.00system 0:33.38elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k

Just over 33 seconds. Better.

From: Arnd Bergmann <arnd@arndb.de>

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.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/include/asm-i386/timex.h b/include/asm-i386/timex.h
index e737729315f4..6d5d757b1b40 100644
--- a/include/asm-i386/timex.h
+++ b/include/asm-i386/timex.h
@@ -7,7 +7,7 @@
 #define _ASMi386_TIMEX_H
 
 #include <linux/config.h>
-#include <asm/msr.h>
+#include <asm/processor.h>
 
 #ifdef CONFIG_X86_ELAN
 #  define CLOCK_TICK_RATE 1189200 /* AMD Elan has different frequency! */
@@ -40,14 +40,17 @@ extern cycles_t cacheflush_time;
 
 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;