RE: [PATCH v3] x86/hyper-v: Fix hv tsc page based sched_clock for hibernation

[Michael Kelley]

From: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx> Sent: Monday, September 16, 2024 10:39 PM
> 
> 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. This makes hv tsc page based sched_clock continuous and ensures
> that post resume, it starts from where it left off during suspend.
> Override x86_platform.save_sched_clock_state and
> x86_platform.restore_sched_clock_state routines to correct this as soon
> as possible.
> 
> 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>
> ---
> Changes from v2:
> https://lore.kernel.org/all/20240911045632.3757-1-namjain@xxxxxxxxxxxxxxxxxxx/
> Addressed Michael's comments:
> * Changed commit msg to include information on making timestamps
>   continuous
> * Changed subject to reflect the new file being changed
> * Changed variable name for saving offset/counters
> * Moved comment on new function introduced from header file to function
>   definition.
> * Removed the equations in comments
> * Rebased to latest linux-next tip
> 
> Changes from v1:
> https://lore.kernel.org/all/20240909053923.8512-1-namjain@xxxxxxxxxxxxxxxxxxx/ 
> * 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     | 58 ++++++++++++++++++++++++++++++
>  drivers/clocksource/hyperv_timer.c | 14 +++++++-
>  include/clocksource/hyperv_timer.h |  2 ++
>  3 files changed, 73 insertions(+), 1 deletion(-)

This all looks good to me now. Thanks for indulging all my comments. :-)

One minor nit: The "Subject:" prefix should not have a dash in "hyper-v".
The goal is to be consistent in the prefixes used for a particular file instead
of wandering all over the place. If you check the commit history for
arch/x86/kernel/cpu/mshyperv.c, you'll see that it is "x86/hyperv", not
"x86/hyperv-v".

This is super picky, so don't respin just for this. The maintainer can
probably fix it when accepting the patch if there are no other changes.

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index ead967479fa6..e8e25d6e64cd 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -224,6 +224,63 @@ static void hv_machine_crash_shutdown(struct pt_regs
> *regs)
>  	hyperv_cleanup();
>  }
>  #endif /* CONFIG_CRASH_DUMP */
> +
> +static u64 hv_ref_counter_at_suspend;
> +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_ref_counter_at_suspend = hv_read_reference_counter();
> +}
> +
> +static void restore_hv_clock_tsc_state(void)
> +{
> +	/*
> +	 * Adjust the offsets used by hv tsc clocksource to
> +	 * account for the time spent before hibernation.
> +	 * adjusted value = reference counter (time) at suspend
> +	 *                - reference counter (time) now.
> +	 */
> +	hv_adj_sched_clock_offset(hv_ref_counter_at_suspend - 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)
> @@ -590,6 +647,7 @@ 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 99177835cade..b39dee7b93af 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
> @@ -470,6 +471,17 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
>  	hv_set_msr(HV_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
>  }
> 
> +/*
> + * 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.
> + */
> +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..aa5233b1eba9 100644
> --- a/include/clocksource/hyperv_timer.h
> +++ b/include/clocksource/hyperv_timer.h
> @@ -38,6 +38,8 @@ 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);
> 
> +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: a430d95c5efa2b545d26a094eb5f624e36732af0
> --
> 2.34.1