pvclock.c

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/*  paravirtual clock -- common code used by kvm/xen

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that 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 <linux/kernel.h>
#include <linux/percpu.h>
#include <asm/pvclock.h>

/*
 * These are perodically updated
 *    xen: magic shared_info page
 *    kvm: gpa registered via msr
 * and then copied here.
 */
struct pvclock_shadow_time {
    u64 tsc_timestamp;     /* TSC at last update of time vals.  */
    u64 system_timestamp;  /* Time, in nanosecs, since boot.    */
    u32 tsc_to_nsec_mul;
    int tsc_shift;
    u32 version;
    u8  flags;
};

static u8 valid_flags __read_mostly = 0;

void pvclock_set_flags(u8 flags)
{
    valid_flags = flags;
}

static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
{
    u64 delta = native_read_tsc() - shadow->tsc_timestamp;
    return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
                   shadow->tsc_shift);
}

/*
 * Reads a consistent set of time-base values from hypervisor,
 * into a shadow data area.
 */
static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
                    struct pvclock_vcpu_time_info *src)
{
    do {
        dst->version = src->version;
        rmb();      /* fetch version before data */
        dst->tsc_timestamp     = src->tsc_timestamp;
        dst->system_timestamp  = src->system_time;
        dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
        dst->tsc_shift         = src->tsc_shift;
        dst->flags             = src->flags;
        rmb();      /* test version after fetching data */
    } while ((src->version & 1) || (dst->version != src->version));

    return dst->version;
}

unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
{
    u64 pv_tsc_khz = 1000000ULL << 32;

    do_div(pv_tsc_khz, src->tsc_to_system_mul);
    if (src->tsc_shift < 0)
        pv_tsc_khz <<= -src->tsc_shift;
    else
        pv_tsc_khz >>= src->tsc_shift;
    return pv_tsc_khz;
}

static atomic64_t last_value = ATOMIC64_INIT(0);

void pvclock_resume(void)
{
    atomic64_set(&last_value, 0);
}

cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
{
    struct pvclock_shadow_time shadow;
    unsigned version;
    cycle_t ret, offset;
    u64 last;

    do {
        version = pvclock_get_time_values(&shadow, src);
        barrier();
        offset = pvclock_get_nsec_offset(&shadow);
        ret = shadow.system_timestamp + offset;
        barrier();
    } while (version != src->version);

    if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
        (shadow.flags & PVCLOCK_TSC_STABLE_BIT))
        return ret;

    /*
     * Assumption here is that last_value, a global accumulator, always goes
     * forward. If we are less than that, we should not be much smaller.
     * We assume there is an error marging we're inside, and then the correction
     * does not sacrifice accuracy.
     *
     * For reads: global may have changed between test and return,
     * but this means someone else updated poked the clock at a later time.
     * We just need to make sure we are not seeing a backwards event.
     *
     * For updates: last_value = ret is not enough, since two vcpus could be
     * updating at the same time, and one of them could be slightly behind,
     * making the assumption that last_value always go forward fail to hold.
     */
    last = atomic64_read(&last_value);
    do {
        if (ret < last)
            return last;
        last = atomic64_cmpxchg(&last_value, last, ret);
    } while (unlikely(last != ret));

    return ret;
}

void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
                struct pvclock_vcpu_time_info *vcpu_time,
                struct timespec *ts)
{
    u32 version;
    u64 delta;
    struct timespec now;

    /* get wallclock at system boot */
    do {
        version = wall_clock->version;
        rmb();      /* fetch version before time */
        now.tv_sec  = wall_clock->sec;
        now.tv_nsec = wall_clock->nsec;
        rmb();      /* fetch time before checking version */
    } while ((wall_clock->version & 1) || (version != wall_clock->version));

    delta = pvclock_clocksource_read(vcpu_time);    /* time since system boot */
    delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;

    now.tv_nsec = do_div(delta, NSEC_PER_SEC);
    now.tv_sec = delta;

    set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
}