[tip: timers/core] vdso: Add comment about reason for vdso struct ordering

["tip-bot2 for Anna-Maria Behnsen" <tip-bot2@linutronix.de>]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     d00106bbdfa82732c23cba44491c38f8c410d865
Gitweb:        https://git.kernel.org/tip/d00106bbdfa82732c23cba44491c38f8c410d865
Author:        Anna-Maria Behnsen <anna-maria@linutronix.de>
AuthorDate:    Mon, 01 Jul 2024 16:47:55 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 03 Jul 2024 21:27:03 +02:00

vdso: Add comment about reason for vdso struct ordering

struct vdso_data is optimized for fast access to the often required struct
members. The optimization is not documented in the struct description but
it should be kept in mind, when working with the vdso_data struct.
                                                                                                                                                                                                                                             
Add a comment to the struct description.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20240701-vdso-cleanup-v1-2-36eb64e7ece2@linutronix.de

---
 include/vdso/datapage.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index d04d394..7647e09 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -77,6 +77,10 @@ struct vdso_timestamp {
  * vdso_data will be accessed by 64 bit and compat code at the same time
  * so we should be careful before modifying this structure.
  *
+ * The ordering of the struct members is optimized to have fast access to the
+ * often required struct members which are related to CLOCK_REALTIME and
+ * CLOCK_MONOTONIC. This information is stored in the first cache lines.
+ *
  * @basetime is used to store the base time for the system wide time getter
  * VVAR page.
  *