Re: [Qemu-devel] [PATCH v2] qemu-ga: Add the guest-suspend command

[Luiz Capitulino]

On Wed, 14 Dec 2011 13:43:23 -0600
Michael Roth <address@hidden> wrote:

> On 12/14/2011 12:17 PM, Luiz Capitulino wrote:
> > On Wed, 14 Dec 2011 14:38:55 -0200
> > Luiz Capitulino<address@hidden>  wrote:
> >
> >>>> I'm also wondering if we could use g_child_watch_add(), but it's not 
> >>>> clear
> >>>> to me if it works with processes not created with g_spawn_*() functions.
> >>>
> >>> GPid's map to something other than PIDs on Windows, so I think we'd have
> >>> issues there. But our fork() approach probably wouldn't work at all on
> >>> Windows except maybe under cygwin, so at some point we'd probably want
> >>> to switch over to g_spawn for this kind of stuff anyway...
> >>>
> >>> So this might be a good point to switch over to using the glib functions.
> >>>
> >>> Would you mind trying to do the hibernate/zombie reaping stuff using
> >>> g_spawn+g_child_watch_add()? It might end up being the easiest route.
> >>> Otherwise I can take a look at it later today.
> >>
> >> Well, there are two problems with g_spawn wrt to the manual method of
> >> writing to the sysfs file. The first one is that I'm not sure if g_spawn()
> >> reports the file not found error synchronously. The other problem is that,
> >> I'd have to fork() anyway to write to the sysfs file (unless we decide that
> >> it's ok to do this synchronously, which seems ok to me).
> >
> > The version below uses g_spawn_async(). The code is a bit simpler than 
> > previous
> > versions, but there are two important details about it:
> >
> >   1. I'm letting g_spawn_async() reap the child automatically. I don't know
> >      how it does it though. I'd guess it uses g_child_watch_add(), worst
> >      case it ignores SIGCHLD (although I think this would be awful)
> 
> Weird, doesn't seem to add a watcher or do anything with SIGCHLD. You're 
> not seeing an error that's causing it to reap it immediately after the fork?

No. I haven't tested it much though, but the basic use case seemed to work
fine.

> >
> >   2. The manual method of writing to the sysfs is done synchronously. This
> >      means that the command response will only be sent when the guest 
> > resumes
> 
> Want to avoid sync blocking calls as much as possible until timeouts are 
> sorted out, particularly in this case. Currently, with hibernate at 
> least, QEMU will exit (or is something being done to change that 
> behavior?), libvirt will restart the VM later via some other interface, 
> establish a connection to a new monitor and new agent channel, then gets 
> a spurious response from the agent for a command it issue to a different 
> QEMU process far in the past. So the common scenario would need nasty, 
> special handling in libvirt and they'd probably hate us for it.

Problem is: even with the other approach there's no way to guarantee that
the response will arrive before the guest suspends or shuts down, as it
depends on which process runs first.

If we want to guarantee that the client will see a response before the
guest suspends, then we need some form of synchronization. Like, the child
should wait for the parent to send the response to the client before
exec()ing (or writing to the sysfs file).

> 
> So probably best to stick with your previous approach for now, and look 
> for a glib solution later.
> 
> >
> > If you think this approach is acceptable, I'll test it more, update its doc,
> > etc and post it again.
> >
> > diff --git a/qapi-schema-guest.json b/qapi-schema-guest.json
> > index 5f8a18d..63f65a6 100644
> > --- a/qapi-schema-guest.json
> > +++ b/qapi-schema-guest.json
> > @@ -219,3 +219,20 @@
> >   ##
> >   { 'command': 'guest-fsfreeze-thaw',
> >     'returns': 'int' }
> > +
> > +##
> > +# @guest-suspend
> > +#
> > +# Suspend guest execution by entering ACPI power state S3 or S4.
> > +#
> > +# @mode: 'hibernate' RAM content is saved in the disk and the guest is
> > +#                    powered down (this corresponds to ACPI S4)
> > +#        'sleep'     execution is suspended but the RAM retains its 
> > contents
> > +#                    (this corresponds to ACPI S3)
> > +#
> > +# Notes: This is an asynchronous request. There's no guarantee it will
> > +# succeed. Errors will be logged to guest's syslog.
> > +#
> > +# Since: 1.1
> > +##
> > +{ 'command': 'guest-suspend', 'data': { 'mode': 'str' } }
> > diff --git a/qga/guest-agent-commands.c b/qga/guest-agent-commands.c
> > index a09c8ca..0c6b78e 100644
> > --- a/qga/guest-agent-commands.c
> > +++ b/qga/guest-agent-commands.c
> > @@ -574,6 +574,56 @@ int64_t qmp_guest_fsfreeze_thaw(Error **err)
> >   }
> >   #endif
> >
> > +#define LINUX_SYS_STATE_FILE "/sys/power/state"
> > +
> > +void qmp_guest_suspend(const char *mode, Error **err)
> > +{
> > +    GError *error = NULL;
> > +    gchar *argv[2];
> > +
> > +    if (strcmp(mode, "hibernate") == 0) {
> > +        argv[0] = (gchar *) "pm-hibernate";
> > +    } else if (strcmp(mode, "sleep") == 0) {
> > +        argv[0] = (gchar *) "pm-suspend";
> > +    } else if (strcmp(mode, "hybrid") == 0) {
> > +        argv[0] = (gchar *) "pm-hybrid";
> > +    } else {
> > +        error_set(err, QERR_INVALID_PARAMETER, "mode");
> > +        return;
> > +    }
> > +
> > +    argv[1] = NULL;
> > +    if (g_spawn_async(NULL, argv, NULL,G_SPAWN_SEARCH_PATH |
> > +                                        G_SPAWN_FILE_AND_ARGV_ZERO,
> > +                                        NULL, NULL, NULL,&error)<  0) {
> > +        int fd;
> > +        const char *cmd;
> > +
> > +        slog("%s\n", error->message);
> > +        g_error_free(error);
> > +
> > +        if (strcmp(mode, "hybrid") == 0) {
> > +            error_set(err, QERR_UNDEFINED_ERROR);
> > +            return;
> > +        }
> > +
> > +        slog("trying to suspend using the manual method...\n");
> > +
> > +        fd = open(LINUX_SYS_STATE_FILE, O_WRONLY);
> > +        if (fd<  0) {
> > +            error_set(err, QERR_OPEN_FILE_FAILED, LINUX_SYS_STATE_FILE);
> > +            return;
> > +        }
> > +
> > +        cmd = strcmp(mode, "sleep") == 0 ? "mem" : "disk";
> > +        if (write(fd, cmd, strlen(cmd))<  0) {
> > +            error_set(err, QERR_IO_ERROR);
> > +        }
> > +
> > +        close(fd);
> > +    }
> > +}
> > +
> >   /* register init/cleanup routines for stateful command groups */
> >   void ga_command_state_init(GAState *s, GACommandState *cs)
> >   {
>