[PULL 4/5] vfio/igd: Require host VGA decode for legacy mode

["Cédric Le Goater" <clg@redhat.com>]

From: Tomita Moeko <tomitamoeko@gmail.com>

Commit a59d06305fff ("vfio/pci: Introduce x-pci-class-code option")
allows user to expose non-VGA IGD device as VGA controller to the
guest. However, legacy mode requires host VGA range access. Check
that GGC.IVD == 0 before enabling legacy mode to ensure IGD is a real
VGA device claiming host VGA ranges.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250723160906.44941-2-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 docs/igd-assign.txt |  1 +
 hw/vfio/igd.c       | 11 ++++++-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/docs/igd-assign.txt b/docs/igd-assign.txt
index af4e8391fc25898906b0b8bf123e1ec44e45efc0..e54040335ba72dc971babb4d776e46d94f342f70 100644
--- a/docs/igd-assign.txt
+++ b/docs/igd-assign.txt
@@ -48,6 +48,7 @@ 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)
+* IGD claims VGA cycles on host (IGD is VGA controller on host)
 * 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 e7a9d1ffc13044dd01ba83b88b2b3a1c24cfbf98..5b1ad1a8048b8753f524059810b1b17015704ac9 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -113,6 +113,7 @@ static int igd_gen(VFIOPCIDevice *vdev)
 #define IGD_BDSM 0x5c /* Base Data of Stolen Memory */
 #define IGD_BDSM_GEN11 0xc0 /* Base Data of Stolen Memory of gen 11 and later */
 
+#define IGD_GMCH_VGA_DISABLE        BIT(1)
 #define IGD_GMCH_GEN6_GMS_SHIFT     3       /* SNB_GMCH in i915 */
 #define IGD_GMCH_GEN6_GMS_MASK      0x1f
 #define IGD_GMCH_GEN8_GMS_SHIFT     8       /* BDW_GMCH in i915 */
@@ -533,12 +534,14 @@ 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)
+     * - IGD claims VGA cycles on host
      * - 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) &&
+        !(gmch & IGD_GMCH_VGA_DISABLE) &&
         !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)))) {
@@ -568,12 +571,10 @@ static bool vfio_pci_igd_config_quirk(VFIOPCIDevice *vdev, Error **errp)
         }
 
         /*
-         * If IGD VGA Disable is clear (expected) and VGA is not already
-         * enabled, try to enable it. Probably shouldn't be using legacy mode
-         * without VGA, but also no point in us enabling VGA if disabled in
-         * hardware.
+         * If VGA is not already enabled, try to enable it. We shouldn't be
+         * using legacy mode without VGA.
          */
-        if (!(gmch & 0x2) && !vdev->vga && !vfio_populate_vga(vdev, &err)) {
+        if (!vdev->vga && !vfio_populate_vga(vdev, &err)) {
             error_setg(&err, "Unable to enable VGA access");
             goto error;
         }
-- 
2.50.1