Re: [Qemu-devel] [PATCH v3 1/2] qmp: adding 'wakeup-suspend-support' in query-target

[Michael Roth]

Quoting Daniel Henrique Barboza (2018-01-03 05:54:54)
> 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>

Looks good to me, but I'm a little concerned that someone might
register a wakeup notifier in the future for some reason other than
actually supporting wakeup (perhaps as an informational thing, or as
as part of some change to general lifecycle event tracking). I'm not
sure it's currently worth adding an explicit flag that users need to
set, but maybe at least some comments around
qemu_register_wakeup_notifier() noting the potential side-effects of
doing such a thing would be warranted.

> ---
>  arch_init.c             | 1 +
>  include/sysemu/sysemu.h | 1 +
>  qapi-schema.json        | 4 +++-
>  vl.c                    | 5 +++++
>  4 files changed, 10 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..871c1f9bd1 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1844,6 +1844,11 @@ 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
>