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    Subject[PATCH 07/12] pvclock: there's no need to copy time_info into shadow
    Date
    We can use the vcpu_time_info as-is, without needing to copy it.  We just
    need to grab a copy of the version number before starting, then we can
    compute the time using the time_info directly. If things changed under
    our feet, then we just go back and do the whole thing again.

    Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    Cc: Avi Kivity <avi@redhat.com>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Glauber Costa <gcosta@redhat.com>
    Cc: Gerd Hoffmann <kraxel@redhat.com>
    ---
    arch/x86/kernel/pvclock.c | 47 +++++---------------------------------------
    1 files changed, 6 insertions(+), 41 deletions(-)

    diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
    index 963f349..e43cd78 100644
    --- a/arch/x86/kernel/pvclock.c
    +++ b/arch/x86/kernel/pvclock.c
    @@ -20,20 +20,6 @@
    #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.
    */
    @@ -71,30 +57,10 @@ static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
    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)
    +static u64 pvclock_get_nsec_offset(const 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;
    + u64 delta = native_read_tsc() - src->tsc_timestamp;
    + return scale_delta(delta, src->tsc_to_system_mul, src->tsc_shift);
    }

    unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
    @@ -111,15 +77,14 @@ unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)

    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);
    + version = src->version;
    rdtsc_barrier();
    - offset = pvclock_get_nsec_offset(&shadow);
    - ret = shadow.system_timestamp + offset;
    + offset = pvclock_get_nsec_offset(src);
    + ret = src->system_time + offset;
    rdtsc_barrier();
    } while (version != src->version);

    --
    1.6.2.5


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