rdrand.c

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/*
 * This file is part of the Linux kernel.
 *
 * Copyright (c) 2011, Intel Corporation
 * Authors: Fenghua Yu <[email protected]>,
 *          H. Peter Anvin <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
 *
 */

#include <asm/processor.h>
#include <asm/archrandom.h>
#include <asm/sections.h>

static int __init x86_rdrand_setup(char *s)
{
    setup_clear_cpu_cap(X86_FEATURE_RDRAND);
    return 1;
}
__setup("nordrand", x86_rdrand_setup);

/* We can't use arch_get_random_long() here since alternatives haven't run */
static inline int rdrand_long(unsigned long *v)
{
    int ok;
    asm volatile("1: " RDRAND_LONG "\n\t"
             "jc 2f\n\t"
             "decl %0\n\t"
             "jnz 1b\n\t"
             "2:"
             : "=r" (ok), "=a" (*v)
             : "0" (RDRAND_RETRY_LOOPS));
    return ok;
}

/*
 * Force a reseed cycle; we are architecturally guaranteed a reseed
 * after no more than 512 128-bit chunks of random data.  This also
 * acts as a test of the CPU capability.
 */
#define RESEED_LOOP ((512*128)/sizeof(unsigned long))

void __cpuinit x86_init_rdrand(struct cpuinfo_x86 *c)
{
#ifdef CONFIG_ARCH_RANDOM
    unsigned long tmp;
    int i, count, ok;

    if (!cpu_has(c, X86_FEATURE_RDRAND))
        return;     /* Nothing to do */

    for (count = i = 0; i < RESEED_LOOP; i++) {
        ok = rdrand_long(&tmp);
        if (ok)
            count++;
    }

    if (count != RESEED_LOOP)
        clear_cpu_cap(c, X86_FEATURE_RDRAND);
#endif
}