x86/time: make do_settime() uses more accurate

As a comment next to one of the invocations states, get_wallclock_time()
can take over a second. The order of evaluation of function arguments is
in principle unspecified; in practice at least gcc looks to be evaluating
them from last to first. Hence with NOW() invoked first, the respective
value passed to do_settime() can be off by over a second (which is in
contrast to __get_cmos_time() attempting to get the time exactly after an
update, i.e. [pretty] precisely at a seconds boundary).

This also addresses a Misra C:2012 rule 13.2 ("The value of an expression
and its persistent side-effects shall be the same under all permitted
evaluation orders") violation each. The violations aren't flagged because
the entire file is (bogusly) excluded from scanning.

Fixes: f64134cdb8 ("x86: Fix time_resume() to notify all domains of wallclock change")
Fixes: 0bfcf984b7 ("x86: Reintroduce clocksource=tsc")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 54ca0aadc8
master date: 2026-05-07 16:10:52 +0200

xen/arch/x86/time.c

@@ -2567,6 +2567,8 @@ __initcall(verify_tsc_reliability);
 /* Late init function (after interrupts are enabled). */
 int __init init_xen_time(void)
 {
+    unsigned long wc;
+
     tsc_check_writability();
 
     open_softirq(TIME_CALIBRATE_SOFTIRQ, local_time_calibration);
@@ -2584,7 +2586,8 @@ int __init init_xen_time(void)
     printk(XENLOG_INFO "Wallclock source: %s\n", wallclock_type_to_string());
 
     /* NB. get_wallclock_time() can take over one second to execute. */
-    do_settime(get_wallclock_time(), 0, NOW());
+    wc = get_wallclock_time();
+    do_settime(wc, 0,  NOW());
 
     /* Finish platform timer initialization. */
     try_platform_timer_tail();
@@ -2737,6 +2740,8 @@ int time_suspend(void)
 
 int time_resume(void)
 {
+    unsigned long wc;
+
     preinit_pit();
 
     resume_platform_timer();
@@ -2748,7 +2753,8 @@ int time_resume(void)
 
     set_timer(&calibration_timer, NOW() + EPOCH);
 
-    do_settime(get_wallclock_time() + cmos_utc_offset, 0, NOW());
+    wc = get_wallclock_time();
+    do_settime(wc + cmos_utc_offset, 0, NOW());
 
     update_vcpu_system_time(current);