[Qemu-devel] [RFC 5/7] qxl-render: call ppm_save on callback

[Alon Levy]

This changes the behavior of the monitor command. After the previous
patch, there is no longer an option of deadlock with virt-manager, but
ppm_save is called too early, before the update has completed. With this
patch it is called at the correct moment, but that means there is a race
between the monitor command completing and the screendump file being saved.

The only solution is to use an asynchronous monitor command. For a
previous round of this see:
 http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg02810.html

Since that's contentious, I'm suggesting we do something that is almost
correct and doesn't hang, instead of correct and hangs. The screendump
user can inotify on the directory and the file if need be. For casual
monitor usage there is no difference.
---
 hw/qxl-render.c |   27 +++++++++++++++++++++++----
 hw/qxl.c        |   15 +++++++++++----
 hw/qxl.h        |    2 +-
 3 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/hw/qxl-render.c b/hw/qxl-render.c
index 7f9fbca..7b120ab 100644
--- a/hw/qxl-render.c
+++ b/hw/qxl-render.c
@@ -75,15 +75,17 @@ typedef struct QXLRenderUpdateData {
     int redraw;
     QXLRect dirty[32];
     QXLRect area;
+    char *filename;
 } QXLRenderUpdateData;
 
-void qxl_render_update(PCIQXLDevice *qxl)
+void qxl_render_update(PCIQXLDevice *qxl, const char *filename)
 {
     VGACommonState *vga = &qxl->vga;
     QXLRect dirty[32];
     void *ptr;
     int redraw = 0;
     QXLRenderUpdateData *data;
+    QXLCookie *cookie;
 
     if (!is_buffer_shared(vga->ds->surface)) {
         dprint(qxl, 1, "%s: restoring shared displaysurface\n", __func__);
@@ -139,11 +141,24 @@ void qxl_render_update(PCIQXLDevice *qxl)
     data->area.right  = qxl->guest_primary.surface.width;
     data->area.top    = 0;
     data->area.bottom = qxl->guest_primary.surface.height;
+    if (filename) {
+        data->filename = g_strdup(filename);
+    }
+    cookie = qxl_cookie_new(QXL_COOKIE_TYPE_RENDER_UPDATE_AREA,
+                                         0,
+                                         (uint64_t)data);
+#if SPICE_INTERFACE_QXL_MINOR >= 1
     qxl_spice_update_area(qxl, 0, &data->area,
                           data->dirty, ARRAY_SIZE(dirty), 1, QXL_ASYNC,
-                          qxl_cookie_new(QXL_COOKIE_TYPE_RENDER_UPDATE_AREA,
-                                         0,
-                                         (uint64_t)data));
+                          cookie);
+#else
+    qxl_spice_update_area(qxl, 0, &data->area,
+                          data->dirty, ARRAY_SIZE(dirty), 1, QXL_SYNC, NULL);
+    qxl_render_update_area_done(qxl, cookie);
+    if (filename) {
+        ppm_save(filename, qxl->ssd.ds->surface);
+    }
+#endif
 }
 
 void qxl_render_update_area_done(PCIQXLDevice *qxl, QXLCookie *cookie)
@@ -171,6 +186,10 @@ void qxl_render_update_area_done(PCIQXLDevice *qxl, 
QXLCookie *cookie)
                    dirty[i].right - dirty[i].left,
                    dirty[i].bottom - dirty[i].top);
     }
+    if (data->filename) {
+        ppm_save(data->filename, qxl->ssd.ds->surface);
+        g_free(data->filename);
+    }
     g_free(data);
 }
 
diff --git a/hw/qxl.c b/hw/qxl.c
index 5563293..2409cb3 100644
--- a/hw/qxl.c
+++ b/hw/qxl.c
@@ -145,12 +145,12 @@ void qxl_spice_update_area(PCIQXLDevice *qxl, uint32_t 
surface_id,
                            uint32_t clear_dirty_region,
                            qxl_async_io async, QXLCookie *cookie)
 {
-    struct QXLRect *area_copy;
     if (async == QXL_SYNC) {
         qxl->ssd.worker->update_area(qxl->ssd.worker, surface_id, area,
                         dirty_rects, num_dirty_rects, clear_dirty_region);
     } else {
 #if SPICE_INTERFACE_QXL_MINOR >= 1
+        struct QXLRect *area_copy;
         if (cookie == NULL) {
             area_copy = g_malloc0(sizeof(*area_copy));
             memcpy(area_copy, area, sizeof(*area));
@@ -1476,7 +1476,7 @@ static void qxl_hw_update(void *opaque)
         break;
     case QXL_MODE_COMPAT:
     case QXL_MODE_NATIVE:
-        qxl_render_update(qxl);
+        qxl_render_update(qxl, NULL);
         break;
     default:
         break;
@@ -1499,8 +1499,15 @@ static void qxl_hw_screen_dump(void *opaque, const char 
*filename)
     switch (qxl->mode) {
     case QXL_MODE_COMPAT:
     case QXL_MODE_NATIVE:
-        qxl_render_update(qxl);
-        ppm_save(filename, qxl->ssd.ds->surface);
+        /*
+         * TODO: if we use an async update_area, to avoid deadlock with
+         * virt-manager, we postpone the saving of the image until the
+         * rendering is done. This means the image isn't guranteed to be
+         * written when we return to the monitor. Fixing this needs an async
+         * monitor command, whatever the implementation of the concept is
+         * called.
+         */
+        qxl_render_update(qxl, filename);
         break;
     case QXL_MODE_VGA:
         vga->screen_dump(vga, filename);
diff --git a/hw/qxl.h b/hw/qxl.h
index 71d5c35..198abdc 100644
--- a/hw/qxl.h
+++ b/hw/qxl.h
@@ -132,7 +132,7 @@ void qxl_log_command(PCIQXLDevice *qxl, const char *ring, 
QXLCommandExt *ext);
 
 /* qxl-render.c */
 void qxl_render_resize(PCIQXLDevice *qxl);
-void qxl_render_update(PCIQXLDevice *qxl);
+void qxl_render_update(PCIQXLDevice *qxl, const char *filename);
 void qxl_render_cursor(PCIQXLDevice *qxl, QXLCommandExt *ext);
 
 void qxl_render_update_area_done(PCIQXLDevice *qxl, QXLCookie *cookie);
-- 
1.7.9