root/arch/x86/kernel/pvclock.c

DEFINITIONS

1. scale_delta
2. pvclock_get_nsec_offset
3. pvclock_get_time_values
4. pvclock_clocksource_read
5. pvclock_read_wallclock

/*  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;
};

/*
 * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
 * yielding a 64-bit result.
 */
static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
{
        u64 product;
#ifdef __i386__
        u32 tmp1, tmp2;
#endif

        if (shift < 0)
                delta >>= -shift;
        else
                delta <<= shift;

#ifdef __i386__
        __asm__ (
                "mul  %5       ; "
                "mov  %4,%%eax ; "
                "mov  %%edx,%4 ; "
                "mul  %5       ; "
                "xor  %5,%5    ; "
                "add  %4,%%eax ; "
                "adc  %5,%%edx ; "
                : "=A" (product), "=r" (tmp1), "=r" (tmp2)
                : "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
#elif __x86_64__
        __asm__ (
                "mul %%rdx ; shrd $32,%%rdx,%%rax"
                : "=a" (product) : "0" (delta), "d" ((u64)mul_frac) );
#else
#error implement me!
#endif

        return product;
}

static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
{
        u64 delta = native_read_tsc() - shadow->tsc_timestamp;
        return 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;
                rmb();          /* test version after fetching data */
        } while ((src->version & 1) || (dst->version != src->version));

        return dst->version;
}

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

        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);

        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);
}