[PATCH] xen: Fix x86 sched_clock() interface for xen

[Juergen Gross <jgross@xxxxxxxx>]

Commit f94c8d11699759 ("sched/clock, x86/tsc: Rework the x86 'unstable'
sched_clock() interface") broke Xen guest time handling across
migration:

[  187.249951] Freezing user space processes ... (elapsed 0.001 seconds) done.
[  187.251137] OOM killer disabled.
[  187.251137] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
[  187.252299] suspending xenstore...
[  187.266987] xen:grant_table: Grant tables using version 1 layout
[18446743811.706476] OOM killer enabled.
[18446743811.706478] Restarting tasks ... done.
[18446743811.720505] Setting capacity to 16777216

Fix that by setting xen_sched_clock_offset at resume time to ensure a
monotonic clock value.

Fixes: f94c8d11699759 ("sched/clock, x86/tsc: Rework the x86 'unstable' sched_clock() interface")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.11
Reported-by: Hans van Kranenburg <hans@xxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/xen/suspend.c |  4 ++++
 arch/x86/xen/time.c    | 11 +++++++++++
 arch/x86/xen/xen-ops.h |  2 ++
 3 files changed, 17 insertions(+)

diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
index 1d83152c761b..45fc9caf3880 100644
--- a/arch/x86/xen/suspend.c
+++ b/arch/x86/xen/suspend.c
@@ -67,6 +67,8 @@ void xen_arch_resume(void)
 {
 	int cpu;
 
+	xen_clocksource_resume();
+
 	on_each_cpu(xen_vcpu_notify_restore, NULL, 1);
 
 	for_each_online_cpu(cpu)
@@ -81,4 +83,6 @@ void xen_arch_suspend(void)
 		xen_pmu_finish(cpu);
 
 	on_each_cpu(xen_vcpu_notify_suspend, NULL, 1);
+
+	xen_clocksource_suspend();
 }
diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 72bf446c3fee..117ce958ffe6 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -32,6 +32,7 @@
 #define TIMER_SLOP	100000
 
 static u64 xen_sched_clock_offset __read_mostly;
+static u64 xen_clock_value_saved;
 
 /* Get the TSC speed from Xen */
 static unsigned long xen_tsc_khz(void)
@@ -54,6 +55,16 @@ static u64 xen_clocksource_read(void)
 	return ret;
 }
 
+void xen_clocksource_suspend(void)
+{
+	xen_clock_value_saved = xen_clocksource_read() - xen_sched_clock_offset;
+}
+
+void xen_clocksource_resume(void)
+{
+	xen_sched_clock_offset = xen_clocksource_read() - xen_clock_value_saved;
+}
+
 static u64 xen_clocksource_get_cycles(struct clocksource *cs)
 {
 	return xen_clocksource_read();
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 0e60bd918695..a17d3bdab6b8 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -63,6 +63,8 @@ void __init xen_build_dynamic_phys_to_machine(void);
 void __init xen_vmalloc_p2m_tree(void);
 
 void xen_init_irq_ops(void);
+void xen_clocksource_suspend(void);
+void xen_clocksource_resume(void);
 void xen_setup_timer(int cpu);
 void xen_setup_runstate_info(int cpu);
 void xen_teardown_timer(int cpu);
-- 
2.16.4