[PATCH 3/3] console: make QMP/HMP screendump run in coroutine

[marcandre.lureau@redhat.com]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

Thanks to the monitors coroutine support, the screendump handler can
trigger a graphic_hw_update(), yield and let the main loop run until
update is done. Then the handler is resumed, and ppm_save() will write
the screen image to disk in the coroutine context (thus non-blocking).

Potentially, during non-blocking write, some new graphic update could
happen, and thus the image may have some glitches. Whether that
behaviour is acceptable is discutable. Allocating new memory may not be
a good idea, as framebuffers can be quite large. Even then, QEMU may
become less responsive as it requires paging in etc.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1230527

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 hmp-commands.hx    |  1 +
 monitor/hmp-cmds.c |  3 ++-
 qapi/ui.json       |  3 ++-
 ui/console.c       | 27 ++++++++++++++++++++++++---
 4 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/hmp-commands.hx b/hmp-commands.hx
index cd068389de..ff2d7aa8f3 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -254,6 +254,7 @@ ERST
         .help       = "save screen from head 'head' of display device 'device' "
                       "into PPM image 'filename'",
         .cmd        = hmp_screendump,
+        .coroutine  = true,
     },
 
 SRST
diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
index 9789f4277f..91608bac6d 100644
--- a/monitor/hmp-cmds.c
+++ b/monitor/hmp-cmds.c
@@ -1756,7 +1756,8 @@ err_out:
     goto out;
 }
 
-void hmp_screendump(Monitor *mon, const QDict *qdict)
+void coroutine_fn
+hmp_screendump(Monitor *mon, const QDict *qdict)
 {
     const char *filename = qdict_get_str(qdict, "filename");
     const char *id = qdict_get_try_str(qdict, "device");
diff --git a/qapi/ui.json b/qapi/ui.json
index 9d6721037f..6c7b33cb72 100644
--- a/qapi/ui.json
+++ b/qapi/ui.json
@@ -98,7 +98,8 @@
 #
 ##
 { 'command': 'screendump',
-  'data': {'filename': 'str', '*device': 'str', '*head': 'int'} }
+  'data': {'filename': 'str', '*device': 'str', '*head': 'int'},
+  'coroutine': true }
 
 ##
 # == Spice
diff --git a/ui/console.c b/ui/console.c
index a56fe0dd26..0118f70d9a 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -168,6 +168,7 @@ struct QemuConsole {
     QEMUFIFO out_fifo;
     uint8_t out_fifo_buf[16];
     QEMUTimer *kbd_timer;
+    CoQueue dump_queue;
 
     QTAILQ_ENTRY(QemuConsole) next;
 };
@@ -263,6 +264,7 @@ static void gui_setup_refresh(DisplayState *ds)
 
 void graphic_hw_update_done(QemuConsole *con)
 {
+    qemu_co_queue_restart_all(&con->dump_queue);
 }
 
 void graphic_hw_update(QemuConsole *con)
@@ -340,8 +342,15 @@ static bool ppm_save(int fd, pixman_image_t *image, Error **errp)
     return true;
 }
 
-void qmp_screendump(const char *filename, bool has_device, const char *device,
-                    bool has_head, int64_t head, Error **errp)
+static void graphic_hw_update_bh(void *con)
+{
+    graphic_hw_update(con);
+}
+
+/* Safety: coroutine-only, concurrent-coroutine safe, main thread only */
+void coroutine_fn
+qmp_screendump(const char *filename, bool has_device, const char *device,
+               bool has_head, int64_t head, Error **errp)
 {
     g_autoptr(pixman_image_t) image = NULL;
     QemuConsole *con;
@@ -366,7 +375,15 @@ void qmp_screendump(const char *filename, bool has_device, const char *device,
         }
     }
 
-    graphic_hw_update(con);
+    if (qemu_co_queue_empty(&con->dump_queue)) {
+        /* Defer the update, it will restart the pending coroutines */
+        aio_bh_schedule_oneshot(qemu_get_aio_context(),
+                                graphic_hw_update_bh, con);
+    }
+    qemu_co_queue_wait(&con->dump_queue, NULL);
+
+    /* All pending coroutines are woken up, while BQL taken, no further graphic
+     * update are possible until it is released, take an image ref before that. */
     surface = qemu_console_surface(con);
     if (!surface) {
         error_setg(errp, "no surface");
@@ -381,6 +398,9 @@ void qmp_screendump(const char *filename, bool has_device, const char *device,
         return;
     }
 
+    /* The image content could potentially be updated as the coroutine yields
+     * and releases the BQL. It could produce corrupted dump, but it should be
+     * otherwise safe. */
     if (!ppm_save(fd, image, errp)) {
         qemu_unlink(filename);
     }
@@ -1297,6 +1317,7 @@ static QemuConsole *new_console(DisplayState *ds, console_type_t console_type,
 
     obj = object_new(TYPE_QEMU_CONSOLE);
     s = QEMU_CONSOLE(obj);
+    qemu_co_queue_init(&s->dump_queue);
     s->head = head;
     object_property_add_link(obj, "device", TYPE_DEVICE,
                              (Object **)&s->device,
-- 
2.28.0