[PATCH] x86/time: make do_settime() uses more accurate

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Fixes: f64134cdb81c ("x86: Fix time_resume() to notify all domains of wallclock 
change")
Fixes: 0bfcf984b727 ("x86: Reintroduce clocksource=tsc")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Of course the time it takes to do all the CMOS reads (or whichever else
wallclock time source is in use) also results in an inaccuracy. For
__get_cmos_time() this might be solvable by having it latch NOW() before
doing the 6 reads, but in particular for efi_get_time() there's hardly
anything we can do.

As to Misra rule 13.2: tagging.ecl lists the rule as clean. I also can't
find any deviation for the two instances fixed here. What am I missing?

For __get_cmos_time(), tangentially: Wouldn't we better use the
century byte if available? As it stands, things will break in 2070. Which
is a long way out, yes, but still. (Of course this would mean a 7th slow
I/O port write/read pair.)

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -2575,6 +2575,8 @@ __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);
@@ -2592,7 +2594,8 @@ 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();
@@ -2745,6 +2748,8 @@ int time_suspend(void)
 
 int time_resume(void)
 {
+    unsigned long wc;
+
     preinit_pit();
 
     resume_platform_timer();
@@ -2756,7 +2761,8 @@ 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);