Re: [RFC PATCH 1/2] suspend: extend S2Idle ops by new notify handler

Rafael J. Wysocki
2022-07-19T18:09:18Z

On Thu, Jul 7, 2022 at 2:56 PM Grzegorz Jaszczyk <jaz@semihalf.com> wrote:
>
> Currently the LPS0 prepare_late callback is aimed to run as the very
> last thing before entering the S2Idle state from LPS0 perspective,
> nevertheless between this call and the system actually entering the
> S2Idle state there are several places where the suspension process could
> be canceled.

And why is this a problem?

The cancellation will occur only if there is a wakeup signal that
would otherwise cause one of the CPUs to exit the idle state. Such a
wakeup signal can appear after calling the new notifier as well, so
why does it make a difference?

> In order to notify VMM about guest entering suspend, extend the S2Idle
> ops by new notify callback, which will be really invoked as a very last
> thing before guest actually enters S2Idle state.

It is not guaranteed that "suspend" (defined as all CPUs entering idle
states) will be actually entered even after this "last step".

> Additionally extend the acpi_s2idle_dev_ops by notify() callback so
> any driver can hook into it and allow to implement its own notification.
>
> Taking advantage of e.g. existing acpi_s2idle_dev_ops's prepare/restore
> hooks is not an option since it will not allow to prevent race
> conditions:
> - VM0 enters s2idle
> - host notes about VM0 is in s2idle
> - host continues with system suspension but in the meantime VM0 exits
> s2idle and sends notification but it is already too late (VM could not
> even send notification on time).

Too late for what?

> Introducing notify() as a very last step before the system enters S2Idle
> together with an assumption that the VMM has control over guest
> resumption allows preventing mentioned races.

How does it do that?

It looks like you want suspend-to-idle to behave like S3 and it won't.

> Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
> ---
> drivers/acpi/x86/s2idle.c | 11 +++++++++++
> include/linux/acpi.h | 1 +
> include/linux/suspend.h | 1 +
> kernel/power/suspend.c | 4 ++++
> 4 files changed, 17 insertions(+)
>
> diff --git a/drivers/acpi/x86/s2idle.c b/drivers/acpi/x86/s2idle.c
> index 2963229062f8..d5aff194c736 100644
> --- a/drivers/acpi/x86/s2idle.c
> +++ b/drivers/acpi/x86/s2idle.c
> @@ -520,10 +520,21 @@ void acpi_s2idle_restore_early(void)
> lps0_dsm_func_mask, lps0_dsm_guid);
> }
>
> +static void acpi_s2idle_notify(void)
> +{
> + struct acpi_s2idle_dev_ops *handler;
> +
> + list_for_each_entry(handler, &lps0_s2idle_devops_head, list_node) {
> + if (handler->notify)
> + handler->notify();
> + }
> +}
> +
> static const struct platform_s2idle_ops acpi_s2idle_ops_lps0 = {
> .begin = acpi_s2idle_begin,
> .prepare = acpi_s2idle_prepare,
> .prepare_late = acpi_s2idle_prepare_late,
> + .notify = acpi_s2idle_notify,
> .wake = acpi_s2idle_wake,
> .restore_early = acpi_s2idle_restore_early,
> .restore = acpi_s2idle_restore,
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 4f82a5bc6d98..b32c4baed99b 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -1068,6 +1068,7 @@ struct acpi_s2idle_dev_ops {
> struct list_head list_node;
> void (*prepare)(void);
> void (*restore)(void);
> + void (*notify)(void);
> };
> int acpi_register_lps0_dev(struct acpi_s2idle_dev_ops *arg);
> void acpi_unregister_lps0_dev(struct acpi_s2idle_dev_ops *arg);
> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> index 70f2921e2e70..16ef7f9d9a03 100644
> --- a/include/linux/suspend.h
> +++ b/include/linux/suspend.h
> @@ -191,6 +191,7 @@ struct platform_s2idle_ops {
> int (*begin)(void);
> int (*prepare)(void);
> int (*prepare_late)(void);
> + void (*notify)(void);
> bool (*wake)(void);
> void (*restore_early)(void);
> void (*restore)(void);
> diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> index 827075944d28..6ba211b94ed1 100644
> --- a/kernel/power/suspend.c
> +++ b/kernel/power/suspend.c
> @@ -100,6 +100,10 @@ static void s2idle_enter(void)
>
> /* Push all the CPUs into the idle loop. */
> wake_up_all_idle_cpus();
> +
> + if (s2idle_ops && s2idle_ops->notify)
> + s2idle_ops->notify();
> +
> /* Make the current CPU wait so it can enter the idle loop too. */
> swait_event_exclusive(s2idle_wait_head,
> s2idle_state == S2IDLE_STATE_WAKE);
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>