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

From: Michael Roth
Subject: Re: [Qemu-devel] [PATCH v2] qemu-ga: Add the guest-suspend command
Date: Wed, 14 Dec 2011 12:06:13 -0600
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:8.0) Gecko/20111105 Thunderbird/8.0

On 12/14/2011 10:38 AM, Luiz Capitulino wrote:
On Wed, 14 Dec 2011 09:54:29 -0600
Michael Roth<address@hidden>  wrote:

On 12/14/2011 07:00 AM, Luiz Capitulino wrote:
On Tue, 13 Dec 2011 14:03:08 -0600
Michael Roth<address@hidden>   wrote:

On 12/13/2011 12:28 PM, Luiz Capitulino wrote:
It supports two modes: "hibernate" (which corresponds to S4) and
"sleep" (which corresponds to S3). It will try to execute the
pm-hibernate or pm-suspend scripts, if the scripts don't exist
the command will try to suspend by directly writing to the
"/sys/power/state" file.

An interesting implementation detail is how to cleanup the child's
status on termination, so that we don't create zombies. I've
choosen to ignore the SIGCHLD signal. This will cause the kernel to
automatically cleanup the child's status on its termination.

One downside to blocking SIGCHLD is it can screw with child processes
that utilize it. `sudo` for instance will just hang indefinitely because
it handles it's own cleanup via SIGCHLD, we might run into similar cases
with various pm-hibernate/pm-suspend implementations as well.

This will also screw with anything we launch via guest-exec as well,
once that goes in.

I wonder if we can tie the qemu_add_child_watch() stuff into qemu-ga's
main loop, or maybe just implement something similar...


    - add a qemu-ga.c:signal_channel_add() that creates a non-blocking
pipe and associate the read-side with a GIOChannel, then ties the
channel into the main loop via g_io_add_watch() on qemu-ga startup, with
an associated callback that reads everything off the pipe, then iterates
through a list of registered pids and does a non-blocking wait(pid, ...)
on each.

    - add a SIGCHLD handler that writes a 1 to the write side of the pipe
whenever it gets called

Is the pipe really needed? Is there any side effect if we call waitpid()
from the signal handler considering it won't block?

In the current state of things, I believe that might actually be
sufficient. I was thinking of the eventual guest-exec case though: we
need to be able to poll for a guest-exec'd process' return status
asynchronously by calling a guest-exec-status command that does the
wait(), so if we use a waitpid(-1, ...) SIGCHLD handler, or block
SIGCHLD, the return status would be intercepted/lost.

What I had in mind was to do what qemu_add_child_watch() does, ie. it
iterates through a list of child pids calling waitpid() on them instead
of doing waitpid(-1,...). The only difference is that we would do it from
a signal handler.

Ah, yah that might work. I'm not sure how well our list functions will behave when accessed via an interrupt handler. We'll have race conditions at least:

#define QTAILQ_INSERT_TAIL(head, elm, field) do {                      \
        (elm)->field.tqe_next = NULL;                                  \
        (elm)->field.tqe_prev = (head)->tqh_last;                      \
        /* interrupt */
        *(head)->tqh_last = (elm);                                     \
        (head)->tqh_last = &(elm)->field.tqe_next;

So if we fork() and the process completes and triggers the interrupt before we add the pid to the list, it will remain a zombie until something else triggers the handler. It's not a huge deal...we at least avoid accumulating zombies, but I'd be weary of other side-effects as well, especially if we also remove entries from the list if they've been waited on successfully.

Maybe that could work for guest-exec too: we could let the signal handler
collect the exit status and store them. Then guest-status could retrieve
the status from there.

Yah, I think we can tie all this together rather nicely with a little work.

We have several options here. Maybe I should do the simplest solution for
the guest-suspend command and you work on improving it for guest-exec.

That's fine, I need to look at this more. But if we're gonna take the simplest approach for now, let's not even bother with the pid registration (and potential races), and just do a waitpid(-1, ...) in a SIGCHLD handler like QEMU did before the child watch stuff was added, and I'll make the changes necessary for the guest-exec stuff before that gets added.

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,

"error can be NULL to ignore errors, or non-NULL to report errors. If an error is set, the function returns FALSE. Errors are reported even if they occur in the child (for example if the executable in argv[0] is not found). Typically the message field of returned errors should be displayed to users. Possible errors are those from the G_SPAWN_ERROR domain."

So I'd think we'd be able to report the FNF synchronously even if we launch into the background.

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).

I think what we'd have to do is open the file, then pass it in as stdout for a g_spawn'd `echo "disk"` command. I'm not sure how we'd manage cleaning up the FD though, if we're doing it asynchronously. And synchronous means every guest-suspend call will eventually time out, so I don't think that's doable right now.

So I'd say keep it the way it is for now. Probably not worth spending too much time on until we see what the Windows stuff looks like anyway.

Then, when creating a child, you just register the child by adding the
pid to the list that the signal channel callback checks.

Signed-off-by: Luiz Capitulino<address@hidden>

I've tested this w/o any virtio driver, as they don't support S4 yet. For
S4 it seems to work ok. I couldn't fully test S3 because we lack a way to
resume from it, but by checking the logs it seems to work fine.



o Rename the command to 'guest-suspend'
o Add 'mode' parameter
o Use pm-utils scripts
o Cleanup child termination status

    qapi-schema-guest.json     |   17 +++++++++++
    qemu-ga.c                  |   11 +++++++-
    qga/guest-agent-commands.c |   64 
    3 files changed, 91 insertions(+), 1 deletions(-)

diff --git a/qapi-schema-guest.json b/qapi-schema-guest.json
index 29989fe..656bde9 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/qemu-ga.c b/qemu-ga.c
index 60d4972..b32e96c 100644
--- a/qemu-ga.c
+++ b/qemu-ga.c
@@ -61,7 +61,7 @@ static void quit_handler(int sig)

    static void register_signal_handlers(void)
-    struct sigaction sigact;
+    struct sigaction sigact, sigact_chld;
        int ret;

        memset(&sigact, 0, sizeof(struct sigaction));
@@ -76,6 +76,15 @@ static void register_signal_handlers(void)
        if (ret == -1) {
            g_error("error configuring signal handler: %s", strerror(errno));
+    /* This should cause the kernel to automatically cleanup child
+       termination status */
+    memset(&sigact_chld, 0, sizeof(struct sigaction));
+    sigact_chld.sa_handler = SIG_IGN;
+    ret = sigaction(SIGCHLD,&sigact_chld, NULL);
+    if (ret == -1) {
+        g_error("error configuring signal handler: %s", strerror(errno));
+    }

    static void usage(const char *cmd)
diff --git a/qga/guest-agent-commands.c b/qga/guest-agent-commands.c
index a09c8ca..4799638 100644
--- a/qga/guest-agent-commands.c
+++ b/qga/guest-agent-commands.c
@@ -574,6 +574,70 @@ int64_t qmp_guest_fsfreeze_thaw(Error **err)

+#define LINUX_PM_UTILS_PATH "/usr/sbin"
+#define LINUX_SYS_STATE_FILE "/sys/power/state"
+void qmp_guest_suspend(const char *mode, Error **err)
+    int ret, fd = -1;
+    const char *pmutils_bin;
+    char pmutils_bin_path[PATH_MAX];
+    if (strcmp(mode, "hibernate") == 0) {
+        pmutils_bin = "pm-hibernate";
+    } else if (strcmp(mode, "sleep") == 0) {
+        pmutils_bin = "pm-suspend";
+    } else {
+        error_set(err, QERR_INVALID_PARAMETER, "mode");
+        return;
+    }
+    snprintf(pmutils_bin_path, sizeof(pmutils_bin_path), "%s/%s",
+             LINUX_PM_UTILS_PATH, pmutils_bin);
+    if (access(pmutils_bin_path, X_OK) != 0) {
+        pmutils_bin = NULL;
+        fd = open(LINUX_SYS_STATE_FILE, O_WRONLY);
+        if (fd<    0) {
+            error_set(err, QERR_OPEN_FILE_FAILED, LINUX_SYS_STATE_FILE);
+            return;
+        }
+    }
+    ret = fork();
+    if (ret == 0) {
+        /* child */
+        setsid();
+        fclose(stdin);
+        fclose(stdout);
+        fclose(stderr);
+        if (pmutils_bin) {
+            ret = execl(pmutils_bin_path, pmutils_bin, NULL);
+            if (ret) {
+                 slog("%s failed: %s", pmutils_bin_path, strerror(errno));
+             }
+        } else {
+            const char *cmd = strcmp(mode, "sleep") == 0 ? "mem" : "disk";
+            ret = write(fd, cmd, strlen(cmd));
+            if (ret<    0) {
+                slog("can't write to %s: %s\n", LINUX_SYS_STATE_FILE,
+                     strerror(errno));
+            }
+            close(fd);
+        }
+        exit(!!ret);
+    } else if (ret<    0) {
+        error_set(err, QERR_UNDEFINED_ERROR);
+        return;
+    }
+    if (!pmutils_bin) {
+        close(fd);
+    }
    /* register init/cleanup routines for stateful command groups */
    void ga_command_state_init(GAState *s, GACommandState *cs)

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