[PULL 10/17] vfio/igd: add x-igd-gms option back to set DSM region size for guest

[Cédric Le Goater]

From: Tomita Moeko <tomitamoeko@gmail.com>

DSM region is likely to store framebuffer in Windows, a small DSM
region may cause display issues (e.g. half of the screen is black).
Since 971ca22f041b ("vfio/igd: don't set stolen memory size to zero"),
the x-igd-gms option was functionally removed, QEMU uses host's
original value, which is determined by DVMT Pre-Allocated option in
Intel FSP of host bios.

However, some vendors do not expose this config item to users. In
such cases, x-igd-gms option can be used to manually set the data
stolen memory size for guest. So this commit brings this option back,
keeping its old behavior. When it is not specified, QEMU uses host's
value.

When DVMT Pre-Allocated option is available in host BIOS, user should
set DSM region size there instead of using x-igd-gms option.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: 20241206122749.9893-11-tomitamoeko@gmail.com">https://lore.kernel.org/r/20241206122749.9893-11-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 hw/vfio/igd.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
index 
73c06bbf64d85b8dbf1575dac34ef8b7e64a3490..b0fef90240ca6dcc0ead7ed536202d618546b4b1
 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -14,6 +14,7 @@
 #include "qemu/units.h"
 #include "qemu/error-report.h"
 #include "qapi/error.h"
+#include "qapi/qmp/qerror.h"
 #include "hw/hw.h"
 #include "hw/nvram/fw_cfg.h"
 #include "pci.h"
@@ -722,6 +723,31 @@ void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
 
     QLIST_INSERT_HEAD(&vdev->bars[nr].quirks, quirk, next);
 
+    /*
+     * Allow user to override dsm size using x-igd-gms option, in multiples of
+     * 32MiB. This option should only be used when the desired size cannot be
+     * set from DVMT Pre-Allocated option in host BIOS.
+     */
+    if (vdev->igd_gms) {
+        if (gen < 8) {
+            if (vdev->igd_gms <= 0x10) {
+                gmch &= ~(IGD_GMCH_GEN6_GMS_MASK << IGD_GMCH_GEN6_GMS_SHIFT);
+                gmch |= vdev->igd_gms << IGD_GMCH_GEN6_GMS_SHIFT;
+            } else {
+                error_report(QERR_INVALID_PARAMETER_VALUE,
+                             "x-igd-gms", "0~0x10");
+            }
+        } else {
+            if (vdev->igd_gms <= 0x40) {
+                gmch &= ~(IGD_GMCH_GEN8_GMS_MASK << IGD_GMCH_GEN8_GMS_SHIFT);
+                gmch |= vdev->igd_gms << IGD_GMCH_GEN8_GMS_SHIFT;
+            } else {
+                error_report(QERR_INVALID_PARAMETER_VALUE,
+                             "x-igd-gms", "0~0x40");
+            }
+        }
+    }
+
     ggms_size = igd_gtt_memory_size(gen, gmch);
     gms_size = igd_stolen_memory_size(gen, gmch);
 
-- 
2.47.1