[Spice-devel] [PATCH] Don't close all but one display during reboot.

[Jonathon Jongsma]

When a guest is rebooted, the QXL driver gets unloaded at some point in
the reboot process. When the driver is unloaded, the spice server sets a
single flag to FALSE: RedWorker::driver_cap_monitors_config. This flag
indicates whether the driver is capable of sending its own monitors config
messages to the client.

The only place this flag is used is when a new primary surface is created. If
this flag is true, the server assumes that the driver will send its own
monitors config very soon after the surface is created. If it's false, the
server directly sends its own temporary monitors config message to the client
since the driver cannot.  This temporary monitors config message is based on
the size of the just-created primary surface and always has a maximum monitor
count of 1.

This flag is initialized to false at startup so that in early boot (e.g. vga
text mode), the server will send out these 'temporary' monitor config messages
to the client whenever the primary surface is destroyed and re-created. This
causes the client to resize its window to match the size of the guest. When the
QXL driver is eventually loaded and starts taking over the monitors config
responsibilities, we set this flag to true and the server stops sending
monitors config messages out on its own.

If we reboot and set this flag to false, it will result in the server sending a
monitors config message to the client indicating that the guest now supports a
maximum of 1 monitor. If the guest happens to have more than one display window
open, it will destroy those extra windows because they exceed the maximum
allowed number of monitors. This means that if you reboot a guest with 4
monitors, after reboot it will only have 1 monitor.

To avoid this, we assume that if we had the ability to support multiple
monitors at some point, that ability will return at some point. So when the
server constructs its own temporary monitors config message to send to the
client (when the driver_cap_monitors_config flag is false), we continue to send
the previous maximum monitor count instead of always sending a maximum of 1.

Resolves: rhbz#1274447
---

NOTE: this fix is a workaround that assumes some things that may not be valid
in all cases.  The main assumption is that if the QXL driver was used before
the reboot, it will continue to be used after the reboot. If this assumption is
violated, the server will continue to report that the guest supports e.g. 4
displays even though the driver may only support 1. The client would then be
able to attempt to enable additional displays, which would inevitably fail
without explanation.  This is not a huge problem, but maybe it's not
acceptable. If it's not acceptable, I think that fixing this bug would require
significantly more work and may not be worth the effort.

 server/display-channel.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/server/display-channel.c b/server/display-channel.c
index a554bfd..88ee194 100644
--- a/server/display-channel.c
+++ b/server/display-channel.c
@@ -2459,15 +2459,18 @@ void display_channel_set_monitors_config_to_primary(DisplayChannel *display)
 {
     DrawContext *context = &display->priv->surfaces[0].context;
     QXLHead head = { 0, };
+    uint16_t old_max = 1;
 
     spice_return_if_fail(display->priv->surfaces[0].context.canvas);
 
-    if (display->priv->monitors_config)
+    if (display->priv->monitors_config) {
+        old_max = display->priv->monitors_config->max_allowed;
         monitors_config_unref(display->priv->monitors_config);
+    }
 
     head.width = context->width;
     head.height = context->height;
-    display->priv->monitors_config = monitors_config_new(&head, 1, 1);
+    display->priv->monitors_config = monitors_config_new(&head, 1, old_max);
 }
 
 void display_channel_reset_image_cache(DisplayChannel *self)
-- 
2.9.3