The gfx_passthru option needs to be enabled whenever IGD is assigned,
otherwise qemu refuses to start. Similarly, if gfx_passthru is enabled,
IGD needs to be assigned, otherwise libxl refuses to start the guest.
This means the gfx_passthru is fully redundant to assigning IGD (besides
enabling various non-bootable configurations).
Change the default value to follow IGD assignment state. For that, use
existing libxl__detect_gfx_passthru_kind (move from libxl_dm.c to
libxl_create.c).

While the option is designed with various GFX in mind, only IGD ever got
a special treatment. PCI passthrough of other GFX devices (some AMD and
Nvidia at least) works just fine without setting gfx_passthru at all.

This change simplifies configuration, but also fixes IGD passthrough
when using libvirt (which doesn't expose gfx_passthru option).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 docs/man/xl.cfg.5.pod.in   |  3 +++
 tools/libxl/libxl_create.c | 27 ++++++++++++++++++++++++++-
 tools/libxl/libxl_dm.c     | 20 +-------------------
 3 files changed, 30 insertions(+), 20 deletions(-)

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index 0532739c1f..32228c1361 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -1250,6 +1250,9 @@ Intel Graphics Device.
 
 =back
 
+By default, this option is enabled if Intel Graphics Device is assigned to the
+VM.
+
 Note that some graphics cards (AMD/ATI cards, for example) do not
 necessarily require the B<gfx_passthru> option, so you can use the normal Xen
 PCI passthrough to assign the graphics card as a secondary graphics
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 75862dc6ed..7aca51cb9c 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -72,6 +72,22 @@ void libxl__rdm_setdefault(libxl__gc *gc, libxl_domain_build_info *b_info)
                             LIBXL_RDM_MEM_BOUNDARY_MEMKB_DEFAULT;
 }
 
+static enum libxl_gfx_passthru_kind
+libxl__detect_gfx_passthru_kind(libxl__gc *gc,
+                                const libxl_domain_config *guest_config)
+{
+    const libxl_domain_build_info *b_info = &guest_config->b_info;
+
+    if (b_info->u.hvm.gfx_passthru_kind != LIBXL_GFX_PASSTHRU_KIND_DEFAULT)
+        return b_info->u.hvm.gfx_passthru_kind;
+
+    if (libxl__is_igd_vga_passthru(gc, guest_config)) {
+        return LIBXL_GFX_PASSTHRU_KIND_IGD;
+    }
+
+    return LIBXL_GFX_PASSTHRU_KIND_DEFAULT;
+}
+
 int libxl__domain_build_info_setdefault(libxl__gc *gc,
                                         libxl_domain_build_info *b_info)
 {
@@ -411,7 +427,8 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
 
         libxl_defbool_setdefault(&b_info->u.hvm.nographic, false);
 
-        libxl_defbool_setdefault(&b_info->u.hvm.gfx_passthru, false);
+        libxl_defbool_setdefault(&b_info->u.hvm.gfx_passthru,
+                b_info->u.hvm.gfx_passthru_kind != LIBXL_GFX_PASSTHRU_KIND_DEFAULT);
 
         libxl__rdm_setdefault(gc, b_info);
         break;
@@ -1177,6 +1194,14 @@ int libxl__domain_config_setdefault(libxl__gc *gc,
             ? libxl__get_required_iommu_memory(d_config->b_info.max_memkb)
             : 0;
 
+    if (d_config->b_info.type == LIBXL_DOMAIN_TYPE_HVM) {
+        if (d_config->b_info.u.hvm.gfx_passthru_kind == LIBXL_GFX_PASSTHRU_KIND_DEFAULT) {
+            /* this may also keep LIBXL_GFX_PASSTHRU_KIND_DEFAULT */
+            d_config->b_info.u.hvm.gfx_passthru_kind =
+                libxl__detect_gfx_passthru_kind(gc, d_config);
+        }
+    }
+
     ret = libxl__domain_build_info_setdefault(gc, &d_config->b_info);
     if (ret) {
         LOGD(ERROR, domid, "Unable to set domain build info defaults");
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index f2dc5696b9..381be5e6ed 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -981,22 +981,6 @@ static char *dm_spice_options(libxl__gc *gc,
     return opt;
 }
 
-static enum libxl_gfx_passthru_kind
-libxl__detect_gfx_passthru_kind(libxl__gc *gc,
-                                const libxl_domain_config *guest_config)
-{
-    const libxl_domain_build_info *b_info = &guest_config->b_info;
-
-    if (b_info->u.hvm.gfx_passthru_kind != LIBXL_GFX_PASSTHRU_KIND_DEFAULT)
-        return b_info->u.hvm.gfx_passthru_kind;
-
-    if (libxl__is_igd_vga_passthru(gc, guest_config)) {
-        return LIBXL_GFX_PASSTHRU_KIND_IGD;
-    }
-
-    return LIBXL_GFX_PASSTHRU_KIND_DEFAULT;
-}
-
 /* colo mode */
 enum {
     LIBXL__COLO_NONE = 0,
@@ -1798,9 +1782,7 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
         }
 
         if (libxl_defbool_val(b_info->u.hvm.gfx_passthru)) {
-            enum libxl_gfx_passthru_kind gfx_passthru_kind =
-                            libxl__detect_gfx_passthru_kind(gc, guest_config);
-            switch (gfx_passthru_kind) {
+            switch (b_info->u.hvm.gfx_passthru_kind) {
             case LIBXL_GFX_PASSTHRU_KIND_IGD:
                 machinearg = GCSPRINTF("%s,igd-passthru=on", machinearg);
                 break;
-- 
2.25.4