[Bug 1888964] Re: Segfault using GTK display with dmabuf (iGVT-g) on Wayland

[Thomas Huth <1888964@bugs.launchpad.net>]

The QEMU project is currently moving its bug tracking to another system.
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closed already. Thus we are setting the bug state to "Incomplete" now.

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Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1888964

Title:
  Segfault using GTK display with dmabuf (iGVT-g) on Wayland

Status in QEMU:
  Incomplete

Bug description:
  When using...
   a) Intel virtualized graphics (iGVT-g) with dmabuf output
   b) QEMU's GTK display with GL output enabled (-display gtk,gl=on)
   c) A Wayland compositor (Sway in my case)
  a segfault occurs at some point on boot (I guess as soon as the guest starts using the virtual graphics card?)

  The origin is the function dpy_gl_scanout_dmabuf in ui/console.c, where it calls
      con->gl->ops->dpy_gl_scanout_dmabuf(con->gl, dmabuf);
  However, the ops field (struct DisplayChangeListenerOps) does not have dpy_gl_scanout_dmabuf set because it is set to dcl_gl_area_ops which does not have dpy_gl_scanout_dmabuf set.
  Only dcl_egl_ops has dpy_gl_scanout_dmabuf set.
  Currently, the GTK display uses EGL on X11 displays, but GtkGLArea on Wayland. This can be observed in early_gtk_display_init() in ui/gtk.c, where it says (simplified code):

  if (opts->has_gl && opts->gl != DISPLAYGL_MODE_OFF) {
          if (GDK_IS_WAYLAND_DISPLAY(gdk_display_get_default())) {
              gtk_use_gl_area = true;
              gtk_gl_area_init();
          } else {
              DisplayGLMode mode = opts->has_gl ? opts->gl : DISPLAYGL_MODE_ON;
              gtk_egl_init(mode);
          }
  }

  To reproduce the findings above, add this assertion to dpy_gl_scanout_dmabuf:
      assert(con->gl->ops->dpy_gl_scanout_dmabuf);
  This will make the segfault turn into an assertion failure.

  A workaround is to force QEMU to use GDK's X11 backend (using
  GDK_BACKEND=x11).

  Note: This might be a duplicate of 1775011, however the information
  provided in that bug report is not sufficient to make the assertion.

  QEMU version: b0ce3f021e0157e9a5ab836cb162c48caac132e1 (from Git master branch)
  OS: Arch Linux, Kernel Version 5.17.0-1

  Relevant flags of the QEMU invocation:
  qemu-system-x86_64 \
    -vga none \
    -device vfio-pci-nohotplug,sysfsdev="$GVT_DEV",romfile="${ROMFILE}",display=on,x-igd-opregion=on,ramfb=on \
    -display gtk,gl=on

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