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: <sta...@vger.kernel.org> # 4.11
Reported-by: Hans van Kranenburg <h...@knorrie.org>
Signed-off-by: Juergen Gross <jgr...@suse.com>
---
 arch/x86/xen/time.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 72bf446c3fee..6e29794573b7 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -361,8 +361,6 @@ void xen_timer_resume(void)
 {
        int cpu;
 
-       pvclock_resume();
-
        if (xen_clockevent != &xen_vcpuop_clockevent)
                return;
 
@@ -379,12 +377,15 @@ static const struct pv_time_ops xen_time_ops __initconst 
= {
 };
 
 static struct pvclock_vsyscall_time_info *xen_clock __read_mostly;
+static u64 xen_clock_value_saved;
 
 void xen_save_time_memory_area(void)
 {
        struct vcpu_register_time_memory_area t;
        int ret;
 
+       xen_clock_value_saved = xen_clocksource_read() - xen_sched_clock_offset;
+
        if (!xen_clock)
                return;
 
@@ -404,7 +405,7 @@ void xen_restore_time_memory_area(void)
        int ret;
 
        if (!xen_clock)
-               return;
+               goto out;
 
        t.addr.v = &xen_clock->pvti;
 
@@ -421,6 +422,11 @@ void xen_restore_time_memory_area(void)
        if (ret != 0)
                pr_notice("Cannot restore secondary vcpu_time_info (err %d)",
                          ret);
+
+out:
+       /* Need pvclock_resume() before using xen_clocksource_read(). */
+       pvclock_resume();
+       xen_sched_clock_offset = xen_clocksource_read() - xen_clock_value_saved;
 }
 
 static void xen_setup_vsyscall_time_info(void)
-- 
2.16.4

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