[PATCH] clocksource: hyper-v: Fix hv tsc page based sched_clock for hibernation

[Naman Jain]

read_hv_sched_clock_tsc() assumes that the Hyper-V clock counter is
bigger than the variable hv_sched_clock_offset, which is cached during
early boot, but depending on the timing this assumption may be false
when a hibernated VM starts again (the clock counter starts from 0
again) and is resuming back (Note: hv_init_tsc_clocksource() is not
called during hibernation/resume); consequently,
read_hv_sched_clock_tsc() may return a negative integer (which is
interpreted as a huge positive integer since the return type is u64)
and new kernel messages are prefixed with huge timestamps before
read_hv_sched_clock_tsc() grows big enough (which typically takes
several seconds).

Fix the issue by saving the Hyper-V clock counter just before the
suspend, and using it to correct the hv_sched_clock_offset in
resume. Override x86_platform.save_sched_clock_state  and
x86_platform.restore_sched_clock_state so that we don't
have to touch the common x86 code.

Note: if Invariant TSC is available, the issue doesn't happen because
1) we don't register read_hv_sched_clock_tsc() for sched clock:
See commit e5313f1c5404 ("clocksource/drivers/hyper-v: Rework
clocksource and sched clock setup");
2) the common x86 code adjusts TSC similarly: see
__restore_processor_state() ->  tsc_verify_tsc_adjust(true) and
x86_platform.restore_sched_clock_state().

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1349401ff1aa ("clocksource/drivers/hyper-v: Suspend/resume Hyper-V clocksource for hibernation")
Co-developed-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx>
---
 drivers/clocksource/hyperv_timer.c | 64 +++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index b2a080647e41..7aa44b8aae2e 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -27,7 +27,10 @@
 #include <asm/mshyperv.h>
 
 static struct clock_event_device __percpu *hv_clock_event;
-static u64 hv_sched_clock_offset __ro_after_init;
+
+/* Can have negative values, after resume from hibernation, so keep them s64 */
+static s64 hv_sched_clock_offset __read_mostly;
+static s64 hv_sched_clock_offset_saved;
 
 /*
  * If false, we're using the old mechanism for stimer0 interrupts
@@ -51,6 +54,9 @@ static int stimer0_irq = -1;
 static int stimer0_message_sint;
 static __maybe_unused DEFINE_PER_CPU(long, stimer0_evt);
 
+static void (*old_save_sched_clock_state)(void);
+static void (*old_restore_sched_clock_state)(void);
+
 /*
  * Common code for stimer0 interrupts coming via Direct Mode or
  * as a VMbus message.
@@ -434,6 +440,39 @@ static u64 noinstr read_hv_sched_clock_tsc(void)
 		(NSEC_PER_SEC / HV_CLOCK_HZ);
 }
 
+/*
+ * Hyper-V clock counter resets during hibernation. Save and restore clock
+ * offset during suspend/resume, while also considering the time passed
+ * before suspend. This is to make sure that sched_clock using hv tsc page
+ * based clocksource, proceeds from where it left off during suspend and
+ * it shows correct time for the timestamps of kernel messages after resume.
+ */
+static void save_hv_clock_tsc_state(void)
+{
+	hv_sched_clock_offset_saved = hv_read_reference_counter();
+}
+
+static void restore_hv_clock_tsc_state(void)
+{
+	/*
+	 * Time passed before suspend = hv_sched_clock_offset_saved
+	 *                            - hv_sched_clock_offset (old)
+	 *
+	 * After Hyper-V clock counter resets, hv_sched_clock_offset needs a correction.
+	 *
+	 * New time = hv_read_reference_counter() (future) - hv_sched_clock_offset (new)
+	 * New time = Time passed before suspend + hv_read_reference_counter() (future)
+	 *                                       - hv_read_reference_counter() (now)
+	 *
+	 * Solving the above two equations gives:
+	 *
+	 * hv_sched_clock_offset (new) = hv_sched_clock_offset (old)
+	 *                             - hv_sched_clock_offset_saved
+	 *                             + hv_read_reference_counter() (now))
+	 */
+	hv_sched_clock_offset -= hv_sched_clock_offset_saved - hv_read_reference_counter();
+}
+
 static void suspend_hv_clock_tsc(struct clocksource *arg)
 {
 	union hv_reference_tsc_msr tsc_msr;
@@ -456,6 +495,24 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
 	hv_set_msr(HV_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
 }
 
+/*
+ * Functions to override save_sched_clock_state and restore_sched_clock_state
+ * functions of x86_platform. The Hyper-V clock counter is reset during
+ * suspend-resume and the offset used to measure time needs to be
+ * corrected, post resume.
+ */
+static void hv_save_sched_clock_state(void)
+{
+	save_hv_clock_tsc_state();
+	old_save_sched_clock_state();
+}
+
+static void hv_restore_sched_clock_state(void)
+{
+	restore_hv_clock_tsc_state();
+	old_restore_sched_clock_state();
+}
+
 #ifdef HAVE_VDSO_CLOCKMODE_HVCLOCK
 static int hv_cs_enable(struct clocksource *cs)
 {
@@ -539,6 +596,11 @@ static void __init hv_init_tsc_clocksource(void)
 
 	hv_read_reference_counter = read_hv_clock_tsc;
 
+	old_save_sched_clock_state = x86_platform.save_sched_clock_state;
+	x86_platform.save_sched_clock_state = hv_save_sched_clock_state;
+	old_restore_sched_clock_state = x86_platform.restore_sched_clock_state;
+	x86_platform.restore_sched_clock_state = hv_restore_sched_clock_state;
+
 	/*
 	 * TSC page mapping works differently in root compared to guest.
 	 * - In guest partition the guest PFN has to be passed to the

base-commit: da3ea35007d0af457a0afc87e84fddaebc4e0b63
-- 
2.25.1