Re: [Qemu-devel] [PATCH v4 1/2] qmp: adding 'wakeup-suspend-support' in query-target

[Michael Roth]

Quoting Daniel Henrique Barboza (2018-01-05 07:03:13)
> When issuing the qmp/hmp 'system_wakeup' command, what happens in a
> nutshell is:
> 
> - qmp_system_wakeup_request set runstate to RUNNING, sets a wakeup_reason
> and notify the event
> - in the main_loop, all vcpus are paused, a system reset is issued, all
> subscribers of wakeup_notifiers receives a notification, vcpus are then
> resumed and the wake up QAPI event is fired
> 
> Note that this procedure alone doesn't ensure that the guest will awake
> from SUSPENDED state - the subscribers of the wake up event must take
> action to resume the guest, otherwise the guest will simply reboot.
> 
> At this moment there are only two subscribers of the wake up event: one
> in hw/acpi/core.c and another one in hw/i386/xen/xen-hvm.c. This means
> that system_wakeup does not work as intended with other architectures.
> 
> However, only the presence of 'system_wakeup' is required for QGA to
> support 'guest-suspend-ram' and 'guest-suspend-hybrid' at this moment.
> This means that the user/management will expect to suspend the guest using
> one of those suspend commands and then resume execution using system_wakeup,
> regardless of the support offered in system_wakeup in the first place.
> 
> This patch adds a new flag called 'wakeup-suspend-support' in TargetInfo
> that allows the caller to query if the guest supports wake up from
> suspend via system_wakeup. It goes over the subscribers of the wake up
> event and, if it's empty, it assumes that the guest does not support
> wake up from suspend (and thus, pm-suspend itself).
> 
> This is the expected output of query-target when running a x86 guest:
> 
> {"execute" : "query-target"}
> {"return": {"arch": "x86_64", "wakeup-suspend-support": true}}
> 
> This is the output when running a pseries guest:
> 
> {"execute" : "query-target"}
> {"return": {"arch": "ppc64", "wakeup-suspend-support": false}}
> 
> Given that the TargetInfo structure is read-only, adding a new flag to
> it is backwards compatible. There is no need to deprecate the old
> TargetInfo format.
> 
> With this extra tool, management can avoid situations where a guest
> that does not have proper suspend/wake capabilities ends up in
> inconsistent state (e.g.
> https://github.com/open-power-host-os/qemu/issues/31).
> 
> Signed-off-by: Daniel Henrique Barboza <address@hidden>
> ---
>  arch_init.c             |  1 +
>  include/sysemu/sysemu.h |  1 +
>  qapi-schema.json        |  4 +++-
>  vl.c                    | 29 +++++++++++++++++++++++++++++
>  4 files changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index a0b8ed6167..9c5c519d9d 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -109,6 +109,7 @@ TargetInfo *qmp_query_target(Error **errp)
>      TargetInfo *info = g_malloc0(sizeof(*info));
> 
>      info->arch = g_strdup(TARGET_NAME);
> +    info->wakeup_suspend_support = !qemu_wakeup_notifier_is_empty();
> 
>      return info;
>  }
> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> index 31612caf10..b15374e0b8 100644
> --- a/include/sysemu/sysemu.h
> +++ b/include/sysemu/sysemu.h
> @@ -69,6 +69,7 @@ typedef enum WakeupReason {
>  void qemu_system_reset_request(ShutdownCause reason);
>  void qemu_system_suspend_request(void);
>  void qemu_register_suspend_notifier(Notifier *notifier);
> +bool qemu_wakeup_notifier_is_empty(void);
>  void qemu_system_wakeup_request(WakeupReason reason);
>  void qemu_system_wakeup_enable(WakeupReason reason, bool enabled);
>  void qemu_register_wakeup_notifier(Notifier *notifier);
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 5c06745c79..2f1c08fde7 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -2388,11 +2388,13 @@
>  # Information describing the QEMU target.
>  #
>  # @arch: the target architecture (eg "x86_64", "i386", etc)
> +# @wakeup-suspend-support: true if the target supports wake up from
> +#                          suspend (since 2.12)
>  #
>  # Since: 1.2.0
>  ##
>  { 'struct': 'TargetInfo',
> -  'data': { 'arch': 'str' } }
> +  'data': { 'arch': 'str', 'wakeup-suspend-support': 'bool' } }
> 
>  ##
>  # @query-target:
> diff --git a/vl.c b/vl.c
> index d3a5c5d021..56d6a1ce5b 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1839,11 +1839,40 @@ void qemu_system_wakeup_enable(WakeupReason reason, 
> bool enabled)
>      }
>  }
> 
> +/* The existence of a wake-up notifier is being checked in the function
> + * qemu_wakeup_notifier_is_empty and it's used in the logic of the
> + * wakeup-suspend-support flag of QMP 'query-target' command. The idea
> + * of this flag is to indicate whether the guest supports wake-up from
> + * suspend (via system_wakeup QMP/HMP call for example), warning the user
> + * that the guest can't handle both wake-up from suspend and the suspend
> + * itself via QGA guest-suspend-ram and guest-suspend-hybrid (if it
> + * can't wake up, it can't be suspended safely).
> + *
> + * The motivation of this design is that this notification will only
> + * fire if a wake up attempt is made by qemu_system_wakeup_request and
> + * the current runstate is RUN_STATE_SUSPENDED. This run state is set by
> + * qemu_system_suspend_request, which at this moment is only being
> + * called in hw/acpi/core.c. This means that, today, only ACPI implements
> + * the proper suspend support and can successfully execute both
> + * guest-suspend-(ram/hybrid) and system_wakeup.
> + *

I think something like the below paragraph gets the important stuff across well
enough; the implementation/design details above make it a bit harder to quickly
grok what's going here (for me at least) and would need to be updated if
new users implement full suspend/wakeup.

Looks good otherwise.

> + * Thus, an assumption is made by the wakeup-suspend-support flag that
> + * only the guests that can go to RUN_STATE_SUSPENDED and wake up properly
> + * would be interested in this wakeup_notifier. Adding a wakeup_notifier
> + * for any other reason will break the logic of the wakeup-suspend-support
> + * flag and can lead to user/management confusion about the suspend/wake-up
> + * support of the guest.
> + */
>  void qemu_register_wakeup_notifier(Notifier *notifier)
>  {
>      notifier_list_add(&wakeup_notifiers, notifier);
>  }
> 
> +bool qemu_wakeup_notifier_is_empty(void)
> +{
> +    return QLIST_EMPTY(&wakeup_notifiers.notifiers);
> +}
> +
>  void qemu_system_killed(int signal, pid_t pid)
>  {
>      shutdown_signal = signal;
> -- 
> 2.13.6
>