perf: Add perf_event_mmap_page::cap_user_time_short ABI

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

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@infradead.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20200716051130.4359-6-leo.yan@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 7b2d6fc..21a1edd 100644 (file)
--- 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.