[PATCH 4/5] perf: Add perf_event_mmap_page::cap_user_time_short ABI

From: Peter Zijlstra
Date: Tue May 12 2020 - 08:47:29 EST


In order to support short clock counters, provide an ABI extention.

As a whole:

u64 time, delta, cyc = read_cycle_counter();

+ if (cap_user_time_short)
+ cyc = time_cycle + ((cyc - time_cycle) & time_mask);

delta = mul_u64_u32_shr(cyc, time_mult, time_shift);

if (cap_user_time_zero)
time = time_zero + delta;

delta += time_offset;

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
include/uapi/linux/perf_event.h | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)

--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -532,9 +532,10 @@ struct perf_event_mmap_page {
cap_bit0_is_deprecated : 1, /* Always 1, signals that bit 0 is zero */

cap_user_rdpmc : 1, /* The RDPMC instruction can be used to read counts */
- cap_user_time : 1, /* The time_* fields are used */
+ cap_user_time : 1, /* The time_{shift,mult,offset} fields are used */
cap_user_time_zero : 1, /* The time_zero field is used */
- cap_____res : 59;
+ cap_user_time_short : 1, /* the time_{cycle,mask} fields are used */
+ cap_____res : 58;
};
};

@@ -593,13 +594,29 @@ struct perf_event_mmap_page {
* ((rem * time_mult) >> time_shift);
*/
__u64 time_zero;
+
__u32 size; /* Header size up to __reserved[] fields. */
+ __u32 __reserved_1;
+
+ /*
+ * If cap_usr_time_short, the hardware clock is less than 64bit wide
+ * and we must compute the 'cyc' value, as used by cap_usr_time, as:
+ *
+ * cyc = time_cycles + ((cyc - time_cycles) & time_mask)
+ *
+ * NOTE: this form is explicitly chosen such that cap_usr_time_short
+ * is a correction on top of cap_usr_time, and code that doesn't
+ * know about cap_usr_time_short still works under the assumption
+ * the counter doesn't wrap.
+ */
+ __u64 time_cycles;
+ __u64 time_mask;

/*
* Hole for extension of the self monitor capabilities
*/

- __u8 __reserved[118*8+4]; /* align to 1k. */
+ __u8 __reserved[116*8]; /* align to 1k. */

/*
* Control data for the mmap() data buffer.