perf/x86: Add ability to calculate TSC from perf sample timestamps

For modern CPUs, perf clock is directly related to TSC.  TSC
can be calculated from perf clock and vice versa using a simple
calculation.  Two of the three componenets of that calculation
are already exported in struct perf_event_mmap_page.  This patch
exports the third.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1372425741-1676-3-git-send-email-adrian.hunter@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 20 insertions, 2 deletions

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 0041aedf2297..efef1d37a371 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -378,7 +378,8 @@ struct perf_event_mmap_page {
                 struct {
                         __u64   cap_usr_time            : 1,
                                 cap_usr_rdpmc           : 1,
-                                cap_____res             : 62;
+                                cap_usr_time_zero       : 1,
+                                cap_____res             : 61;
                 };
         };
 
@@ -420,12 +421,29 @@ struct perf_event_mmap_page {
         __u16   time_shift;
         __u32   time_mult;
         __u64   time_offset;
+        /*
+         * If cap_usr_time_zero, the hardware clock (e.g. TSC) can be calculated
+         * from sample timestamps.
+         *
+         *   time = timestamp - time_zero;
+         *   quot = time / time_mult;
+         *   rem  = time % time_mult;
+         *   cyc = (quot << time_shift) + (rem << time_shift) / time_mult;
+         *
+         * And vice versa:
+         *
+         *   quot = cyc >> time_shift;
+         *   rem  = cyc & ((1 << time_shift) - 1);
+         *   timestamp = time_zero + quot * time_mult +
+         *               ((rem * time_mult) >> time_shift);
+         */
+        __u64   time_zero;
 
                 /*
                  * Hole for extension of the self monitor capabilities
                  */
 
-        __u64   __reserved[120];        /* align to 1k */
+        __u64   __reserved[119];        /* align to 1k */
 
         /*
          * Control data for the mmap() data buffer.