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vfio/igd: Require host VGA decode for legacy mode

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Commit a59d06305f ("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>
This commit is contained in:
Tomita Moeko 2025-07-24 00:09:05 +08:00 • committed by Cédric Le Goater
parent 9751377c3a
commit e0b33efe2a
2 changed files with 7 additions and 5 deletions
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1
docs/igd-assign.txt
View file

@ -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

11
hw/vfio/igd.c
View file

@ -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;
}