diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
index e40e601026..b449316ec0 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -116,6 +116,54 @@ typedef struct VFIOIGDQuirk {
 #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_GEN6_GMS_SHIFT     3
+#define IGD_GMCH_GEN6_GMS_MASK      0x1f
+#define IGD_GMCH_GEN6_GGMS_SHIFT    8
+#define IGD_GMCH_GEN6_GGMS_MASK     0x3
+#define IGD_GMCH_GEN8_GMS_SHIFT     8
+#define IGD_GMCH_GEN8_GMS_MASK      0xff
+#define IGD_GMCH_GEN8_GGMS_SHIFT    6
+#define IGD_GMCH_GEN8_GGMS_MASK     0x3
+
+static uint64_t igd_gtt_memory_size(int gen, uint16_t gmch)
+{
+    uint64_t ggms;
+
+    if (gen < 8) {
+        ggms = (gmch >> IGD_GMCH_GEN6_GGMS_SHIFT) & IGD_GMCH_GEN6_GGMS_MASK;
+    } else {
+        ggms = (gmch >> IGD_GMCH_GEN8_GGMS_SHIFT) & IGD_GMCH_GEN8_GGMS_MASK;
+        ggms *= 2;
+    }
+
+    return ggms * MiB;
+}
+
+static uint64_t igd_stolen_memory_size(int gen, uint32_t gmch)
+{
+    uint64_t gms;
+
+    if (gen < 8) {
+        gms = (gmch >> IGD_GMCH_GEN6_GMS_SHIFT) & IGD_GMCH_GEN6_GMS_MASK;
+    } else {
+        gms = (gmch >> IGD_GMCH_GEN8_GMS_SHIFT) & IGD_GMCH_GEN8_GMS_MASK;
+    }
+
+    if (gen < 9) {
+        if (gms < 0x10) {
+            return gms * 32 * MiB;
+        }
+    } else {
+        if (gms < 0xf0) {
+            return gms * 32 * MiB;
+        } else {
+            return (gms - 0xf0 + 1) * 4 * MiB;
+        }
+    }
+
+    error_report("Unsupported IGD GMS value 0x%lx", gms);
+    return 0;
+}
 
 /*
  * The rather short list of registers that we copy from the host devices.
@@ -264,17 +312,10 @@ static int vfio_pci_igd_lpc_init(VFIOPCIDevice *vdev,
 static int vfio_igd_gtt_max(VFIOPCIDevice *vdev)
 {
     uint32_t gmch = vfio_pci_read_config(&vdev->pdev, IGD_GMCH, sizeof(gmch));
-    int ggms, gen = igd_gen(vdev);
-
-    gmch = vfio_pci_read_config(&vdev->pdev, IGD_GMCH, sizeof(gmch));
-    ggms = (gmch >> (gen < 8 ? 8 : 6)) & 0x3;
-    if (gen > 7) {
-        ggms = 1 << ggms;
-    }
-
-    ggms *= MiB;
+    int gen = igd_gen(vdev);
+    uint64_t ggms_size = igd_gtt_memory_size(gen, gmch);
 
-    return (ggms / (4 * KiB)) * (gen < 8 ? 4 : 8);
+    return (ggms_size / (4 * KiB)) * (gen < 8 ? 4 : 8);
 }
 
 /*
@@ -481,30 +522,6 @@ 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 (gen < 9) {
-        if (gms > 0x10) {
-            error_report("Unsupported IGD GMS value 0x%x", gms);
-            return 0;
-        }
-        return gms * 32;
-    } else {
-        if (gms < 0xf0)
-            return gms * 32;
-        else
-            return (gms - 0xf0) * 4 + 4;
-    }
-}
-
 void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
 {
     g_autofree struct vfio_region_info *rom = NULL;
@@ -514,7 +531,8 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
     VFIOQuirk *quirk;
     VFIOIGDQuirk *igd;
     PCIDevice *lpc_bridge;
-    int i, ret, ggms_mb, gms_mb = 0, gen;
+    int i, ret, gen;
+    uint64_t ggms_size, gms_size;
     uint64_t *bdsm_size;
     uint32_t gmch;
     uint16_t cmd_orig, cmd;
@@ -676,13 +694,8 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
 
     QLIST_INSERT_HEAD(&vdev->bars[nr].quirks, quirk, next);
 
-    /* Determine the size of stolen memory needed for GTT */
-    ggms_mb = (gmch >> (gen < 8 ? 8 : 6)) & 0x3;
-    if (gen > 7) {
-        ggms_mb = 1 << ggms_mb;
-    }
-
-    gms_mb = igd_get_stolen_mb(gen, gmch);
+    ggms_size = igd_gtt_memory_size(gen, gmch);
+    gms_size = igd_stolen_memory_size(gen, gmch);
 
     /*
      * Request reserved memory for stolen memory via fw_cfg.  VM firmware
@@ -693,7 +706,7 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
      * config offset 0x5C.
      */
     bdsm_size = g_malloc(sizeof(*bdsm_size));
-    *bdsm_size = cpu_to_le64((ggms_mb + gms_mb) * MiB);
+    *bdsm_size = cpu_to_le64(ggms_size + gms_size);
     fw_cfg_add_file(fw_cfg_find(), "etc/igd-bdsm-size",
                     bdsm_size, sizeof(*bdsm_size));
 
@@ -744,5 +757,5 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
                      vdev->vbasedev.name);
     }
 
-    trace_vfio_pci_igd_bdsm_enabled(vdev->vbasedev.name, ggms_mb + gms_mb);
+    trace_vfio_pci_igd_bdsm_enabled(vdev->vbasedev.name, *bdsm_size / MiB);
 }
