[PATCH v2] clocksource: hyper-v: Fix hv tsc page based sched_clock for hibernation

Naman Jain
2024-09-11T04:56:44Z

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.

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@vger.kernel.org
Fixes: 1349401ff1aa ("clocksource/drivers/hyper-v: Suspend/resume Hyper-V clocksource for hibernation")
Co-developed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
---
Changes from v1:
https://lore.kernel.org/all/20240909053923.8512-1-namjain@linux.microsoft.com/
* Reorganized code as per Michael's comment, and moved the logic to x86
specific files, to keep hyperv_timer.c arch independent.

---
arch/x86/kernel/cpu/mshyperv.c | 70 ++++++++++++++++++++++++++++++
drivers/clocksource/hyperv_timer.c | 8 +++-
include/clocksource/hyperv_timer.h | 8 ++++
3 files changed, 85 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e0fd57a8ba84..d83a694e387c 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -224,6 +224,75 @@ static void hv_machine_crash_shutdown(struct pt_regs *regs)
hyperv_cleanup();
}
#endif /* CONFIG_CRASH_DUMP */
+
+static u64 hv_sched_clock_offset_saved;
+static void (*old_save_sched_clock_state)(void);
+static void (*old_restore_sched_clock_state)(void);
+
+/*
+ * 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)
+{
+ /*
+ * hv_sched_clock_offset = offset that is used by hyperv_timer clocksource driver
+ * to get time.
+ * 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_adj_sched_clock_offset(hv_sched_clock_offset_saved - hv_read_reference_counter());
+}
+
+/*
+ * 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)
+{
+ old_save_sched_clock_state();
+ save_hv_clock_tsc_state();
+}
+
+static void hv_restore_sched_clock_state(void)
+{
+ restore_hv_clock_tsc_state();
+ old_restore_sched_clock_state();
+}
+
+static void __init x86_setup_ops_for_tsc_pg_clock(void)
+{
+ if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
+ return;
+
+ 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;
+}
#endif /* CONFIG_HYPERV */

static uint32_t __init ms_hyperv_platform(void)
@@ -575,6 +644,7 @@ static void __init ms_hyperv_init_platform(void)

/* Register Hyper-V specific clocksource */
hv_init_clocksource();
+ x86_setup_ops_for_tsc_pg_clock();
hv_vtl_init_platform();
#endif
/*
diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index b2a080647e41..e424892444ed 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -27,7 +27,8 @@
#include <asm/mshyperv.h>

static struct clock_event_device __percpu *hv_clock_event;
-static u64 hv_sched_clock_offset __ro_after_init;
+/* Note: offset can hold negative values after hibernation. */
+static u64 hv_sched_clock_offset __read_mostly;

/*
* If false, we're using the old mechanism for stimer0 interrupts
@@ -456,6 +457,11 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
hv_set_msr(HV_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
}

+void hv_adj_sched_clock_offset(u64 offset)
+{
+ hv_sched_clock_offset -= offset;
+}
+
#ifdef HAVE_VDSO_CLOCKMODE_HVCLOCK
static int hv_cs_enable(struct clocksource *cs)
{
diff --git a/include/clocksource/hyperv_timer.h b/include/clocksource/hyperv_timer.h
index 6cdc873ac907..62e2bad754c0 100644
--- a/include/clocksource/hyperv_timer.h
+++ b/include/clocksource/hyperv_timer.h
@@ -38,6 +38,14 @@ extern void hv_remap_tsc_clocksource(void);
extern unsigned long hv_get_tsc_pfn(void);
extern struct ms_hyperv_tsc_page *hv_get_tsc_page(void);

+/*
+ * Called during resume from hibernation, from overridden
+ * x86_platform.restore_sched_clock_state routine. This is to adjust offsets
+ * used to calculate time for hv tsc page based sched_clock, to account for
+ * time spent before hibernation.
+ */
+extern void hv_adj_sched_clock_offset(u64 offset);
+
static __always_inline bool
hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
u64 *cur_tsc, u64 *time)

base-commit: da3ea35007d0af457a0afc87e84fddaebc4e0b63
--
2.25.1