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perf: Add perf_event_mmap_page::cap_user_time_short ABI

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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>
This commit is contained in:
Peter Zijlstra 2020-07-16 13:11:28 +08:00 committed by Will Deacon
parent 279a811eb5
commit 6c0246a458
1 changed files with 20 additions and 3 deletions
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23
include/uapi/linux/perf_event.h
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@ -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.