Do not allocate grants for framebuffer by default

If glamor is not used (which is default), FBBasePriv seems to be unused.
GUI agent doesn't share the root window with GUI daemon. If glamor is
used, FBBasePriv probably unused in practice too, but at this time I'm not
100% sure about it (see qubes_create_screen_resources() - the "pixmap"
of the root window is passed to glamor, and it isn't clear if it
wouldn't end up used in some window that is handled by the GUI agent).

Based on this observation, if glamor is not used, use normal memory for
the framebuffer. The main benefit is not having the framebuffer
mlock()-ed, which helps especially with small VMs. But also, allocating
it as normal userspace memory gives the kernel more flexibility in
finding memory for it (it doesn't need to be physically continuous,
etc).

Fixes QubesOS/qubes-issues#9992

Author: marmarek

xf86-video-dummy/src/dummy_driver.c

@@ -912,7 +912,7 @@ qubes_create_screen_resources(ScreenPtr pScreen) {
 
     Bool ret = dPtr->CreateScreenResources(pScreen);
 
-    if (ret) {
+    if (ret && dPtr->FBBasePriv) {
         PixmapPtr pixmap = pScreen->GetScreenPixmap(pScreen);
         if (dPtr->glamor)
            glamor_egl_create_textured_pixmap_from_gbm_bo(pixmap, dPtr->front_bo, FALSE);
@@ -1015,10 +1015,17 @@ DUMMYScreenInit(SCREEN_INIT_ARGS_DECL)
         return FALSE;
     }
 
-    dPtr->FBBasePriv = qubes_alloc_pixmap_private(pScrn->videoRam * 1024);
-    if (dPtr->FBBasePriv == NULL)
-        return FALSE;
-    dPtr->FBBase = (void *) dPtr->FBBasePriv->data;
+    if (dPtr->glamor) {
+        dPtr->FBBasePriv = qubes_alloc_pixmap_private(pScrn->videoRam * 1024);
+        if (dPtr->FBBasePriv == NULL)
+            return FALSE;
+        dPtr->FBBase = (void *) dPtr->FBBasePriv->data;
+    } else {
+        dPtr->FBBase = calloc(1, pScrn->videoRam * 1024);
+        if (dPtr->FBBase == NULL)
+            return FALSE;
+    }
+
 
     /*
      * next we save the current state and setup the first mode
@@ -1290,8 +1297,12 @@ DUMMYCloseScreen(CLOSE_SCREEN_ARGS_DECL)
         if (dPtr->FBBasePriv) {
             xf86_qubes_free_pixmap_private(dPtr->FBBasePriv);
             dPtr->FBBasePriv = NULL;
-            dPtr->FBBase = NULL;
+        } else {
+            if (dPtr->FBBase) {
+                free(dPtr->FBBase);
+            }
         }
+        dPtr->FBBase = NULL;
     }
 
     if (dPtr->CursorInfo)