Intel only provides legacy VBIOS for IGD up to Gen9, and there is no
CSM support on later devices. Additionally, Seabios can only handle
32-bit BDSM register used until Gen9. Since legacy mode requires VGA
capability, restrict it to Gen6 through Gen9 devices.

Link: https://lore.kernel.org/qemu-devel/20250325172239.27926-1-tomitamoeko@gmail.com/T/
Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
---
 docs/igd-assign.txt | 1 +
 hw/vfio/igd.c       | 6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/docs/igd-assign.txt b/docs/igd-assign.txt
index 3aed7956d5..95beb76812 100644
--- a/docs/igd-assign.txt
+++ b/docs/igd-assign.txt
@@ -47,6 +47,7 @@ Intel document [1] shows how to dump VBIOS to file. For UEFI Option ROM, see
 
 QEMU also provides a "Legacy" mode that implicitly enables full functionality
 on IGD, it is automatically enabled when
+* IGD generation is 6 to 9 (Sandy Bridge to Comet Lake)
 * Machine type is i440fx
 * IGD is assigned to guest BDF 00:02.0
 * ROM BAR or romfile is present
diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
index d7e4728fdc..e06484c911 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -517,11 +517,13 @@ static bool vfio_pci_igd_config_quirk(VFIOPCIDevice *vdev, Error **errp)
 
     /*
      * For backward compatibility, enable legacy mode when
+     * - Device geneation is 6 to 9 (including both)
      * - Machine type is i440fx (pc_piix)
      * - IGD device is at guest BDF 00:02.0
      * - Not manually disabled by x-igd-legacy-mode=off
      */
     if ((vdev->igd_legacy_mode != ON_OFF_AUTO_OFF) &&
+        (gen >= 6 && gen <= 9) &&
         !strcmp(MACHINE_GET_CLASS(qdev_get_machine())->family, "pc_piix") &&
         (&vdev->pdev == pci_find_device(pci_device_root_bus(&vdev->pdev),
         0, PCI_DEVFN(0x2, 0)))) {
@@ -566,7 +568,9 @@ static bool vfio_pci_igd_config_quirk(VFIOPCIDevice *vdev, Error **errp)
         vdev->features |= VFIO_FEATURE_ENABLE_IGD_LPC;
     } else if (vdev->igd_legacy_mode == ON_OFF_AUTO_ON) {
         error_setg(&err,
-                   "Machine is not i440fx or assigned BDF is not 00:02.0");
+                   "Machine is not i440fx, assigned BDF is not 00:02.0, "
+                   "or device %04x (gen %d) doesn't support legacy mode",
+                   vdev->device_id, gen);
         goto error;
     }
 
-- 
2.47.2