via-rng.c

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/*
 * RNG driver for VIA RNGs
 *
 * Copyright 2005 (c) MontaVista Software, Inc.
 *
 * with the majority of the code coming from:
 *
 * Hardware driver for the Intel/AMD/VIA Random Number Generators (RNG)
 * (c) Copyright 2003 Red Hat Inc <[email protected]>
 *
 * derived from
 *
 * Hardware driver for the AMD 768 Random Number Generator (RNG)
 * (c) Copyright 2001 Red Hat Inc
 *
 * derived from
 *
 * Hardware driver for Intel i810 Random Number Generator (RNG)
 * Copyright 2000,2001 Jeff Garzik <[email protected]>
 * Copyright 2000,2001 Philipp Rumpf <[email protected]>
 *
 * This file is licensed under  the terms of the GNU General Public
 * License version 2. This program is licensed "as is" without any
 * warranty of any kind, whether express or implied.
 */

#include <crypto/padlock.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/hw_random.h>
#include <linux/delay.h>
#include <asm/io.h>
#include <asm/msr.h>
#include <asm/cpufeature.h>
#include <asm/i387.h>




enum {
    VIA_STRFILT_CNT_SHIFT   = 16,
    VIA_STRFILT_FAIL    = (1 << 15),
    VIA_STRFILT_ENABLE  = (1 << 14),
    VIA_RAWBITS_ENABLE  = (1 << 13),
    VIA_RNG_ENABLE      = (1 << 6),
    VIA_NOISESRC1       = (1 << 8),
    VIA_NOISESRC2       = (1 << 9),
    VIA_XSTORE_CNT_MASK = 0x0F,

    VIA_RNG_CHUNK_8     = 0x00, /* 64 rand bits, 64 stored bits */
    VIA_RNG_CHUNK_4     = 0x01, /* 32 rand bits, 32 stored bits */
    VIA_RNG_CHUNK_4_MASK    = 0xFFFFFFFF,
    VIA_RNG_CHUNK_2     = 0x02, /* 16 rand bits, 32 stored bits */
    VIA_RNG_CHUNK_2_MASK    = 0xFFFF,
    VIA_RNG_CHUNK_1     = 0x03, /* 8 rand bits, 32 stored bits */
    VIA_RNG_CHUNK_1_MASK    = 0xFF,
};

/*
 * Investigate using the 'rep' prefix to obtain 32 bits of random data
 * in one insn.  The upside is potentially better performance.  The
 * downside is that the instruction becomes no longer atomic.  Due to
 * this, just like familiar issues with /dev/random itself, the worst
 * case of a 'rep xstore' could potentially pause a cpu for an
 * unreasonably long time.  In practice, this condition would likely
 * only occur when the hardware is failing.  (or so we hope :))
 *
 * Another possible performance boost may come from simply buffering
 * until we have 4 bytes, thus returning a u32 at a time,
 * instead of the current u8-at-a-time.
 *
 * Padlock instructions can generate a spurious DNA fault, so
 * we have to call them in the context of irq_ts_save/restore()
 */

static inline u32 xstore(u32 *addr, u32 edx_in)
{
    u32 eax_out;
    int ts_state;

    ts_state = irq_ts_save();

    asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
        : "=m" (*addr), "=a" (eax_out), "+d" (edx_in), "+D" (addr));

    irq_ts_restore(ts_state);
    return eax_out;
}

static int via_rng_data_present(struct hwrng *rng, int wait)
{
    char buf[16 + PADLOCK_ALIGNMENT - STACK_ALIGN] __attribute__
        ((aligned(STACK_ALIGN)));
    u32 *via_rng_datum = (u32 *)PTR_ALIGN(&buf[0], PADLOCK_ALIGNMENT);
    u32 bytes_out;
    int i;

    /* We choose the recommended 1-byte-per-instruction RNG rate,
     * for greater randomness at the expense of speed.  Larger
     * values 2, 4, or 8 bytes-per-instruction yield greater
     * speed at lesser randomness.
     *
     * If you change this to another VIA_CHUNK_n, you must also
     * change the ->n_bytes values in rng_vendor_ops[] tables.
     * VIA_CHUNK_8 requires further code changes.
     *
     * A copy of MSR_VIA_RNG is placed in eax_out when xstore
     * completes.
     */

    for (i = 0; i < 20; i++) {
        *via_rng_datum = 0; /* paranoia, not really necessary */
        bytes_out = xstore(via_rng_datum, VIA_RNG_CHUNK_1);
        bytes_out &= VIA_XSTORE_CNT_MASK;
        if (bytes_out || !wait)
            break;
        udelay(10);
    }
    rng->priv = *via_rng_datum;
    return bytes_out ? 1 : 0;
}

static int via_rng_data_read(struct hwrng *rng, u32 *data)
{
    u32 via_rng_datum = (u32)rng->priv;

    *data = via_rng_datum;

    return 1;
}

static int via_rng_init(struct hwrng *rng)
{
    struct cpuinfo_x86 *c = &cpu_data(0);
    u32 lo, hi, old_lo;

    /* VIA Nano CPUs don't have the MSR_VIA_RNG anymore.  The RNG
     * is always enabled if CPUID rng_en is set.  There is no
     * RNG configuration like it used to be the case in this
     * register */
    if ((c->x86 == 6) && (c->x86_model >= 0x0f)) {
        if (!cpu_has_xstore_enabled) {
            printk(KERN_ERR PFX "can't enable hardware RNG "
                "if XSTORE is not enabled\n");
            return -ENODEV;
        }
        return 0;
    }

    /* Control the RNG via MSR.  Tread lightly and pay very close
     * close attention to values written, as the reserved fields
     * are documented to be "undefined and unpredictable"; but it
     * does not say to write them as zero, so I make a guess that
     * we restore the values we find in the register.
     */
    rdmsr(MSR_VIA_RNG, lo, hi);

    old_lo = lo;
    lo &= ~(0x7f << VIA_STRFILT_CNT_SHIFT);
    lo &= ~VIA_XSTORE_CNT_MASK;
    lo &= ~(VIA_STRFILT_ENABLE | VIA_STRFILT_FAIL | VIA_RAWBITS_ENABLE);
    lo |= VIA_RNG_ENABLE;
    lo |= VIA_NOISESRC1;

    /* Enable secondary noise source on CPUs where it is present. */

    /* Nehemiah stepping 8 and higher */
    if ((c->x86_model == 9) && (c->x86_mask > 7))
        lo |= VIA_NOISESRC2;

    /* Esther */
    if (c->x86_model >= 10)
        lo |= VIA_NOISESRC2;

    if (lo != old_lo)
        wrmsr(MSR_VIA_RNG, lo, hi);

    /* perhaps-unnecessary sanity check; remove after testing if
       unneeded */
    rdmsr(MSR_VIA_RNG, lo, hi);
    if ((lo & VIA_RNG_ENABLE) == 0) {
        printk(KERN_ERR PFX "cannot enable VIA C3 RNG, aborting\n");
        return -ENODEV;
    }

    return 0;
}


static struct hwrng via_rng = {
    .name       = "via",
    .init       = via_rng_init,
    .data_present   = via_rng_data_present,
    .data_read  = via_rng_data_read,
};


static int __init mod_init(void)
{
    int err;

    if (!cpu_has_xstore)
        return -ENODEV;
    printk(KERN_INFO "VIA RNG detected\n");
    err = hwrng_register(&via_rng);
    if (err) {
        printk(KERN_ERR PFX "RNG registering failed (%d)\n",
               err);
        goto out;
    }
out:
    return err;
}

static void __exit mod_exit(void)
{
    hwrng_unregister(&via_rng);
}

module_init(mod_init);
module_exit(mod_exit);

MODULE_DESCRIPTION("H/W RNG driver for VIA CPU with PadLock");
MODULE_LICENSE("GPL");