The stolen memory is required for the GOP (EFI) driver and the Windows
driver. While the GOP driver seems to work with any stolen memory size,
the Windows driver will crash if the size doesn't match the size
allocated by the host BIOS. For that reason, it doesn't make sense to
overwrite the stolen memory size. It's true that this wastes some VM
memory. In the worst case, the stolen memory can take up more than a GB.
However, that's uncommon. Additionally, it's likely that a bunch of RAM
is assigned to VMs making use of GPU passthrough.

Signed-off-by: Corvin Köhne <c.koehne@beckhoff.com>
---
 hw/vfio/igd.c | 35 ++++++++++++++++++-----------------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
index 8a41b16421..0751c43eae 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -478,6 +478,23 @@ void vfio_probe_igd_bar0_quirk(VFIOPCIDevice *vdev, int nr)
     QLIST_INSERT_HEAD(&vdev->bars[nr].quirks, quirk, next);
 }
 
+static int igd_get_stolen_mb(int gen, uint32_t gmch)
+{
+    int gms;
+
+    if (gen < 8) {
+        gms = (gmch >> 3) & 0x1f;
+    } else {
+        gms = (gmch >> 8) & 0xff;
+    }
+
+    if (gms > 0x10) {
+        error_report("Unsupported IGD GMS value 0x%x", gms);
+        return 0;
+    }
+    return gms * 32;
+}
+
 void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
 {
     g_autofree struct vfio_region_info *rom = NULL;
@@ -655,23 +672,7 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
         ggms_mb = 1 << ggms_mb;
     }
 
-    /*
-     * Assume we have no GMS memory, but allow it to be overridden by device
-     * option (experimental).  The spec doesn't actually allow zero GMS when
-     * when IVD (IGD VGA Disable) is clear, but the claim is that it's unused,
-     * so let's not waste VM memory for it.
-     */
-    gmch &= ~((gen < 8 ? 0x1f : 0xff) << (gen < 8 ? 3 : 8));
-
-    if (vdev->igd_gms) {
-        if (vdev->igd_gms <= 0x10) {
-            gms_mb = vdev->igd_gms * 32;
-            gmch |= vdev->igd_gms << (gen < 8 ? 3 : 8);
-        } else {
-            error_report("Unsupported IGD GMS value 0x%x", vdev->igd_gms);
-            vdev->igd_gms = 0;
-        }
-    }
+    gms_mb = igd_get_stolen_mb(gen, gmch);
 
     /*
      * Request reserved memory for stolen memory via fw_cfg.  VM firmware
-- 
2.46.0