vfio/igd: Do not include GTT stolen size in etc/igd-bdsm-size

Though GTT Stolen Memory (GSM) is right below Data Stolen Memory (DSM)
in host address space, direct access to GSM is prohibited, and it is
not mapped to guest address space. Both host and guest accesses GSM
indirectly through the second half of MMIO BAR0 (GTTMMADR).

Guest firmware only need to reserve a memory region for DSM and program
the BDSM register with the base address of that region, that's actually
what both SeaBIOS[1] and IgdAssignmentDxe does now.

[1] https://gitlab.com/qemu-project/seabios/-/blob/1.12-stable/src/fw/pciinit.c#L319-332

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Link: https://lore.kernel.org/qemu-devel/20250306180131.32970-3-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/igd.c

@@ -112,28 +112,8 @@ static int igd_gen(VFIOPCIDevice *vdev)
 
 #define IGD_GMCH_GEN6_GMS_SHIFT     3       /* SNB_GMCH in i915 */
 #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       /* BDW_GMCH in i915 */
 #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;
-        if (ggms != 0) {
-            ggms = 1ULL << ggms;
-        }
-    }
-
-    return ggms * MiB;
-}
 
 static uint64_t igd_stolen_memory_size(int gen, uint32_t gmch)
 {
@@ -367,7 +347,7 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
     g_autofree struct vfio_region_info *lpc = NULL;
     PCIDevice *lpc_bridge;
     int ret, gen;
-    uint64_t ggms_size, gms_size;
+    uint64_t gms_size;
     uint64_t *bdsm_size;
     uint32_t gmch;
     Error *err = NULL;
@@ -527,7 +507,6 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
         }
     }
 
-    ggms_size = igd_gtt_memory_size(gen, gmch);
     gms_size = igd_stolen_memory_size(gen, gmch);
 
     /*
@@ -539,7 +518,7 @@ 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_size + gms_size);
+    *bdsm_size = cpu_to_le64(gms_size);
     fw_cfg_add_file(fw_cfg_find(), "etc/igd-bdsm-size",
                     bdsm_size, sizeof(*bdsm_size));
 
@@ -559,6 +538,5 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
         pci_set_quad(vdev->emulated_config_bits + IGD_BDSM_GEN11, ~0);
     }
 
-    trace_vfio_pci_igd_bdsm_enabled(vdev->vbasedev.name,
-                                    (ggms_size + gms_size) / MiB);
+    trace_vfio_pci_igd_bdsm_enabled(vdev->vbasedev.name, (gms_size / MiB));
 }