[PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO

[Matt Rickard]

Process clock_gettime(CLOCK_TAI) in vDSO.
This makes the call about as fast as CLOCK_REALTIME and CLOCK_MONOTONIC:

  nanoseconds
 before after clockname
   ---- ----- ---------
    233    87 CLOCK_TAI
     96    93 CLOCK_REALTIME
     88    87 CLOCK_MONOTONIC

Signed-off-by: Matt Rickard <m...@softrans.com.au>
---
 arch/x86/entry/vdso/vclock_gettime.c    | 25 +++++++++++++++++++++++++
 arch/x86/entry/vsyscall/vsyscall_gtod.c |  2 ++
 arch/x86/include/asm/vgtod.h            |  1 +
 3 files changed, 28 insertions(+)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c 
b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d95c60..91ed1bb2a3bb 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -246,6 +246,27 @@ notrace static int __always_inline do_monotonic(struct 
timespec *ts)
        return mode;
 }
 
+notrace static int __always_inline do_tai(struct timespec *ts)
+{
+       unsigned long seq;
+       u64 ns;
+       int mode;
+
+       do {
+               seq = gtod_read_begin(gtod);
+               mode = gtod->vclock_mode;
+               ts->tv_sec = gtod->tai_time_sec;
+               ns = gtod->wall_time_snsec;
+               ns += vgetsns(&mode);
+               ns >>= gtod->shift;
+       } while (unlikely(gtod_read_retry(gtod, seq)));
+
+       ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
+       ts->tv_nsec = ns;
+
+       return mode;
+}
+
 notrace static void do_realtime_coarse(struct timespec *ts)
 {
        unsigned long seq;
@@ -277,6 +298,10 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct 
timespec *ts)
                if (do_monotonic(ts) == VCLOCK_NONE)
                        goto fallback;
                break;
+       case CLOCK_TAI:
+               if (do_tai(ts) == VCLOCK_NONE)
+                       goto fallback;
+               break;
        case CLOCK_REALTIME_COARSE:
                do_realtime_coarse(ts);
                break;
diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c 
b/arch/x86/entry/vsyscall/vsyscall_gtod.c
index e1216dd95c04..d61392fe17f6 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -53,6 +53,8 @@ void update_vsyscall(struct timekeeper *tk)
        vdata->monotonic_time_snsec     = tk->tkr_mono.xtime_nsec
                                        + ((u64)tk->wall_to_monotonic.tv_nsec
                                                << tk->tkr_mono.shift);
+       vdata->tai_time_sec             = tk->xtime_sec
+                                       + tk->tai_offset;
        while (vdata->monotonic_time_snsec >=
                                        (((u64)NSEC_PER_SEC) << 
tk->tkr_mono.shift)) {
                vdata->monotonic_time_snsec -=
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9f0449..adc9f7b20b9c 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -32,6 +32,7 @@ struct vsyscall_gtod_data {
        gtod_long_t     wall_time_coarse_nsec;
        gtod_long_t     monotonic_time_coarse_sec;
        gtod_long_t     monotonic_time_coarse_nsec;
+       gtod_long_t     tai_time_sec;
 
        int             tz_minuteswest;
        int             tz_dsttime;