The default qemu upstream "xenfv" machine type that is used when an HVM
guest is configured for Intel IGD passthrough assigns slot 2 to the
xen platform pci device. It is a requirement that slot 2 be assigned to
the Intel IGD when it is passed through as the primary graphics adapter.
Using the "pc" machine type instead of the "xenfv" machine type in that
case makes it possible for qemu upstream to assign slot 2 to the IGD.

Using the qemu "pc" machine and adding the xen platform device on the
qemu command line instead of using the qemu "xenfv" machine which
automatically adds the xen platform device earlier in the guest creation
process does come with some degredation of startup performance: startup
is slower and some vga drivers in use during early boot are unable to
display the screen at the native resolution of the monitor, but once
the guest operating system (Windows or Linux) is fully loaded, there
is no noticeable difference in the performance of the guest when using
the "pc" machine type instead of the "xenfv" machine type.

With this patch, libxl continues to use default "xenfv" machine type
with the default settings of xen_platform_pci enabled and igd
gfx_passthru disabled. The patch only affects machines configured with
gfx_passthru enabled.

Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
---
Reviewers might find this patch easier to review by looking at the
resulting code in the patched file instead of looking at the diff because
it is hard to follow the logical flow of the resulting code in the diff
because the patch moves the check for igd gfx_passthru before the check
for disabling the xen platform device. That change was made because it
results in a more simplified logical flow in the resulting code.

 tools/libs/light/libxl_dm.c | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
index fc264a3a13..2048815611 100644
--- a/tools/libs/light/libxl_dm.c
+++ b/tools/libs/light/libxl_dm.c
@@ -1809,7 +1809,26 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
             flexarray_append(dm_args, b_info->extra_pv[i]);
         break;
     case LIBXL_DOMAIN_TYPE_HVM:
-        if (!libxl_defbool_val(b_info->u.hvm.xen_platform_pci)) {
+        if (libxl_defbool_val(b_info->u.hvm.gfx_passthru)) {
+            enum libxl_gfx_passthru_kind gfx_passthru_kind =
+                            libxl__detect_gfx_passthru_kind(gc, guest_config);
+            switch (gfx_passthru_kind) {
+            case LIBXL_GFX_PASSTHRU_KIND_IGD:
+                /*
+                 * Using the machine "pc" because with the default machine "xenfv"
+                 * the xen-platform device will be assigned to slot 2, but with
+                 * GFX_PASSTHRU_KIND_IGD, slot 2 needs to be reserved for the Intel IGD.
+                 */
+                machinearg = libxl__strdup(gc, "pc,accel=xen,suppress-vmdesc=on,igd-passthru=on");
+                break;
+            case LIBXL_GFX_PASSTHRU_KIND_DEFAULT:
+                LOGD(ERROR, guest_domid, "unable to detect required gfx_passthru_kind");
+                return ERROR_FAIL;
+            default:
+                LOGD(ERROR, guest_domid, "invalid value for gfx_passthru_kind");
+                return ERROR_INVAL;
+            }
+        } else if (!libxl_defbool_val(b_info->u.hvm.xen_platform_pci)) {
             /* Switching here to the machine "pc" which does not add
              * the xen-platform device instead of the default "xenfv" machine.
              */
@@ -1831,22 +1850,6 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
             }
         }
 
-        if (libxl_defbool_val(b_info->u.hvm.gfx_passthru)) {
-            enum libxl_gfx_passthru_kind gfx_passthru_kind =
-                            libxl__detect_gfx_passthru_kind(gc, guest_config);
-            switch (gfx_passthru_kind) {
-            case LIBXL_GFX_PASSTHRU_KIND_IGD:
-                machinearg = GCSPRINTF("%s,igd-passthru=on", machinearg);
-                break;
-            case LIBXL_GFX_PASSTHRU_KIND_DEFAULT:
-                LOGD(ERROR, guest_domid, "unable to detect required gfx_passthru_kind");
-                return ERROR_FAIL;
-            default:
-                LOGD(ERROR, guest_domid, "invalid value for gfx_passthru_kind");
-                return ERROR_INVAL;
-            }
-        }
-
         flexarray_append(dm_args, machinearg);
         for (i = 0; b_info->extra_hvm && b_info->extra_hvm[i] != NULL; i++)
             flexarray_append(dm_args, b_info->extra_hvm[i]);
-- 
2.39.0