arch/x86/kernel/cpu/rdrand.c

   1 /*
   2  * This file is part of the Linux kernel.
   3  *
   4  * Copyright (c) 2011, Intel Corporation
   5  * Authors: Fenghua Yu <fenghua.yu@intel.com>,
   6  *          H. Peter Anvin <hpa@linux.intel.com>
   7  *
   8  * This program is free software; you can redistribute it and/or modify it
   9  * under the terms and conditions of the GNU General Public License,
  10  * version 2, as published by the Free Software Foundation.
  11  *
  12  * This program is distributed in the hope it will be useful, but WITHOUT
  13  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
  15  * more details.
  16  *
  17  * You should have received a copy of the GNU General Public License along with
  18  * this program; if not, write to the Free Software Foundation, Inc.,
  19  * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  */
  22
  23 #include <asm/processor.h>
  24 #include <asm/archrandom.h>
  25 #include <asm/sections.h>
  26
  27 static int __init x86_rdrand_setup(char *s)
  28 {
  29         setup_clear_cpu_cap(X86_FEATURE_RDRAND);
  30         return 1;
  31 }
  32 __setup("nordrand", x86_rdrand_setup);
  33
  34 /* We can't use arch_get_random_long() here since alternatives haven't run */
  35 static inline int rdrand_long(unsigned long *v)
  36 {
  37         int ok;
  38         asm volatile("1: " RDRAND_LONG "\n\t"
  39                      "jc 2f\n\t"
  40                      "decl %0\n\t"
  41                      "jnz 1b\n\t"
  42                      "2:"
  43                      : "=r" (ok), "=a" (*v)
  44                      : "0" (RDRAND_RETRY_LOOPS));
  45         return ok;
  46 }
  47
  48 /*
  49  * Force a reseed cycle; we are architecturally guaranteed a reseed
  50  * after no more than 512 128-bit chunks of random data.  This also
  51  * acts as a test of the CPU capability.
  52  */
  53 #define RESEED_LOOP ((512*128)/sizeof(unsigned long))
  54
  55 void __cpuinit x86_init_rdrand(struct cpuinfo_x86 *c)
  56 {
  57 #ifdef CONFIG_ARCH_RANDOM
  58         unsigned long tmp;
  59         int i, count, ok;
  60
  61         if (!cpu_has(c, X86_FEATURE_RDRAND))
  62                 return;         /* Nothing to do */
  63
  64         for (count = i = 0; i < RESEED_LOOP; i++) {
  65                 ok = rdrand_long(&tmp);
  66                 if (ok)
  67                         count++;
  68         }
  69
  70         if (count != RESEED_LOOP)
  71                 clear_cpu_cap(c, X86_FEATURE_RDRAND);
  72 #endif
  73 }