[PATCH 0/2] xen/pt: fix FTBFS and reserve PCI slot 2 for the Intel IGD

[PATCH 0/2] xen/pt: fix FTBFS and reserve PCI slot 2 for the Intel IGD

[Chuck Zmudzinski]

From: Chuck Zmudzinski <brchuckz@netscape.net>

This is a series of two patches:

The first fixes FTBFS when --enable-xen and --disable-xen-pci-passthrough
configure options are set with when building for the linux target os.

The second fixes a regression that was introduced many years ago with the
upgrade from the Qemu traditional device model that is still available
from xenbits.xen.org and based on very old Qemu version 0.10.2.

The regression is that the Qemu traditional device model reserves slot 2
for the Intel IGD on the PCI bus when the Intel IGD is passed through
to a Xen HVM domain, but the current Qemu upsream device model does not
and in fact results in a different slot assigned to the Intel IGD.

This behavior does not conform to the requirement that the Intel IGD must
be assigned to slot 2, as noted in docs/igd-assign.txt in the Qemu source
code: "IGD must be given address 02.0 on the PCI root bus in the VM."

I have used the second patch of the series for the past two years with
no problems. Without the patch, the reliability of PCI passthrough of the
Intel IGD to a Xen HVM guest is very poor, and in some cases the guest
fails to start without the patch.

Chuck Zmudzinski (2):
  xen/pt: fix syntax error that causes FTBFS in some configurations
  xen/pt: reserve PCI slot 2 for Intel igd-passthru

 hw/i386/pc_piix.c    |  3 +++
 hw/xen/meson.build   |  2 +-
 hw/xen/xen_pt.c      | 25 +++++++++++++++++++++++++
 hw/xen/xen_pt.h      | 16 ++++++++++++++++
 hw/xen/xen_pt_stub.c |  4 ++++
 5 files changed, 49 insertions(+), 1 deletion(-)

-- 
2.37.2

[PATCH 1/2] xen/pt: fix syntax error that causes FTBFS in some configurations

[Chuck Zmudzinski]

From: Chuck Zmudzinski <brchuckz@netscape.net>

When Qemu is built with --enable-xen and --disable-xen-pci-passthrough
and the target os is linux, the build fails with:

meson.build:3477:2: ERROR: File xen_pt_stub.c does not exist.

Fixes: 582ea95f5f93 ("meson: convert hw/xen")

Signed-off-by: Chuck Zmudzinski <brchuckz@netscape.net>
---
 hw/xen/meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/xen/meson.build b/hw/xen/meson.build
index 08dc1f6857..ae0ace3046 100644
--- a/hw/xen/meson.build
+++ b/hw/xen/meson.build
@@ -18,7 +18,7 @@ if have_xen_pci_passthrough
     'xen_pt_msi.c',
   ))
 else
-  xen_specific_ss.add('xen_pt_stub.c')
+  xen_specific_ss.add(files('xen_pt_stub.c'))
 endif
 
 specific_ss.add_all(when: ['CONFIG_XEN', xen], if_true: xen_specific_ss)
-- 
2.37.2

[PATCH 2/2] xen/pt: reserve PCI slot 2 for Intel igd-passthru

[Chuck Zmudzinski]

From: Chuck Zmudzinski <brchuckz@netscape.net>

Intel specifies that the Intel IGD must occupy slot 2 on the PCI bus,
as noted in docs/igd-assign.txt in the Qemu source code.

Currently, when the xl toolstack is used to configure a Xen HVM guest with
Intel IGD passthrough to the guest with the Qemu upstream device model,
a Qemu emulated PCI device will occupy slot 2 and the Intel IGD will occupy
a different slot. This problem often prevents the guest from booting.

The only available workaround is not good: Configure Xen HVM guests to use
the old and no longer maintained Qemu traditional device model available
from xenbits.xen.org which does reserve slot 2 for the Intel IGD.

To implement this feature in the Qemu upstream device model for Xen HVM
guests, introduce the following new class, functions, types, and macros:

* XEN_PT_DEVICE_CLASS declaration, based on the existing TYPE_XEN_PT_DEVICE
* XEN_PT_DEVICE_GET_CLASS macro helper function for XEN_PT_DEVICE_CLASS
* typedef XenPTQdevRealize function pointer
* XEN_PCI_IGD_SLOT_MASK, the value of slot_reserved_mask to reserve slot 2
* xen_igd_reserve_slot and xen_igd_clear_slot functions

The new xen_igd_reserve_slot function uses the existing slot_reserved_mask
member of PCIBus to reserve PCI slot 2 for Xen HVM guests configured using
the xl toolstack with the gfx_passthru option enabled, which sets the
igd-passthru=on option to Qemu for the Xen HVM machine type.

The new xen_igd_reserve_slot function also needs to be implemented in
hw/xen/xen_pt_stub.c to prevent FTBFS during the link stage for the case
when Qemu is configured with --enable-xen and --disable-xen-pci-passthrough,
in which case it does nothing.

The new xen_igd_clear_slot function overrides qdev->realize of the parent
PCI device class to enable the Intel IGD to occupy slot 2 on the PCI bus
since slot 2 was reserved by xen_igd_reserve_slot when the PCI bus was
created in hw/i386/pc_piix.c for the case when igd-passthru=on.

Signed-off-by: Chuck Zmudzinski <brchuckz@netscape.net>
---
Notes that might be helpful to reviewers of patched code in hw/xen:

The new functions and types are based on recommendations from Qemu docs:
https://qemu.readthedocs.io/en/latest/devel/qom.html

Notes that might be helpful to reviewers of patched code in hw/i386:

The small patch to hw/i386/pc_piix.c is protected by CONFIG_XEN so it does
not affect builds that do not have CONFIG_XEN defined.

xen_igd_gfx_pt_enabled() in the patched hw/i386/pc_piix.c file is an
existing function that is only true when Qemu is built with
xen-pci-passthrough enabled and the administrator has configured the Xen
HVM guest with Qemu's igd-passthru=on option.

 hw/i386/pc_piix.c    |  3 +++
 hw/xen/xen_pt.c      | 25 +++++++++++++++++++++++++
 hw/xen/xen_pt.h      | 16 ++++++++++++++++
 hw/xen/xen_pt_stub.c |  4 ++++
 4 files changed, 48 insertions(+)

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 0b1a79c0fa..a0f04ad62e 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -402,6 +402,9 @@ static void pc_xen_hvm_init(MachineState *machine)
     }
 
     pc_xen_hvm_init_pci(machine);
+    if (xen_igd_gfx_pt_enabled()) {
+        xen_igd_reserve_slot(pcms->bus);
+    }
     pci_create_simple(pcms->bus, -1, "xen-platform");
 }
 #endif
diff --git a/hw/xen/xen_pt.c b/hw/xen/xen_pt.c
index 0ec7e52183..ec42f47fac 100644
--- a/hw/xen/xen_pt.c
+++ b/hw/xen/xen_pt.c
@@ -950,11 +950,35 @@ static void xen_pci_passthrough_instance_init(Object *obj)
     PCI_DEVICE(obj)->cap_present |= QEMU_PCI_CAP_EXPRESS;
 }
 
+void xen_igd_reserve_slot(PCIBus *pci_bus)
+{
+    XEN_PT_LOG(0, "Reserving PCI slot 2 for IGD\n");
+    pci_bus->slot_reserved_mask |= XEN_PCI_IGD_SLOT_MASK;
+}
+
+static void xen_igd_clear_slot(DeviceState *qdev, Error **errp)
+{
+    PCIDevice *pci_dev = (PCIDevice *)qdev;
+    XenPCIPassthroughState *s = XEN_PT_DEVICE(pci_dev);
+    XenPTDeviceClass *xptc = XEN_PT_DEVICE_GET_CLASS(s);
+    PCIBus *pci_bus = pci_get_bus(pci_dev);
+
+    if (is_igd_vga_passthrough(&s->real_device) &&
+        (s->real_device.vendor_id == PCI_VENDOR_ID_INTEL)) {
+        pci_bus->slot_reserved_mask &= ~XEN_PCI_IGD_SLOT_MASK;
+        XEN_PT_LOG(pci_dev, "Intel IGD found, using slot 2\n");
+    }
+    xptc->pci_qdev_realize(qdev, errp);
+}
+
 static void xen_pci_passthrough_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
     PCIDeviceClass *k = PCI_DEVICE_CLASS(klass);
 
+    XenPTDeviceClass *xptc = XEN_PT_DEVICE_CLASS(klass);
+    xptc->pci_qdev_realize = dc->realize;
+    dc->realize = xen_igd_clear_slot;
     k->realize = xen_pt_realize;
     k->exit = xen_pt_unregister_device;
     k->config_read = xen_pt_pci_read_config;
@@ -977,6 +1001,7 @@ static const TypeInfo xen_pci_passthrough_info = {
     .instance_size = sizeof(XenPCIPassthroughState),
     .instance_finalize = xen_pci_passthrough_finalize,
     .class_init = xen_pci_passthrough_class_init,
+    .class_size = sizeof(XenPTDeviceClass),
     .instance_init = xen_pci_passthrough_instance_init,
     .interfaces = (InterfaceInfo[]) {
         { INTERFACE_CONVENTIONAL_PCI_DEVICE },
diff --git a/hw/xen/xen_pt.h b/hw/xen/xen_pt.h
index e7c4316a7d..40b31b5263 100644
--- a/hw/xen/xen_pt.h
+++ b/hw/xen/xen_pt.h
@@ -3,6 +3,7 @@
 
 #include "hw/xen/xen_common.h"
 #include "hw/pci/pci.h"
+#include "hw/pci/pci_bus.h"
 #include "xen-host-pci-device.h"
 #include "qom/object.h"
 
@@ -41,7 +42,20 @@ typedef struct XenPTReg XenPTReg;
 #define TYPE_XEN_PT_DEVICE "xen-pci-passthrough"
 OBJECT_DECLARE_SIMPLE_TYPE(XenPCIPassthroughState, XEN_PT_DEVICE)
 
+#define XEN_PT_DEVICE_CLASS(klass) \
+    OBJECT_CLASS_CHECK(XenPTDeviceClass, klass, TYPE_XEN_PT_DEVICE)
+#define XEN_PT_DEVICE_GET_CLASS(obj) \
+    OBJECT_GET_CLASS(XenPTDeviceClass, obj, TYPE_XEN_PT_DEVICE)
+
+typedef void (*XenPTQdevRealize)(DeviceState *qdev, Error **errp);
+
+typedef struct XenPTDeviceClass {
+    PCIDeviceClass parent_class;
+    XenPTQdevRealize pci_qdev_realize;
+} XenPTDeviceClass;
+
 uint32_t igd_read_opregion(XenPCIPassthroughState *s);
+void xen_igd_reserve_slot(PCIBus *pci_bus);
 void igd_write_opregion(XenPCIPassthroughState *s, uint32_t val);
 void xen_igd_passthrough_isa_bridge_create(XenPCIPassthroughState *s,
                                            XenHostPCIDevice *dev);
@@ -76,6 +90,8 @@ typedef int (*xen_pt_conf_byte_read)
 
 #define XEN_PCI_INTEL_OPREGION 0xfc
 
+#define XEN_PCI_IGD_SLOT_MASK 0x4UL /* Intel IGD slot_reserved_mask */
+
 typedef enum {
     XEN_PT_GRP_TYPE_HARDWIRED = 0,  /* 0 Hardwired reg group */
     XEN_PT_GRP_TYPE_EMU,            /* emul reg group */
diff --git a/hw/xen/xen_pt_stub.c b/hw/xen/xen_pt_stub.c
index 2d8cac8d54..5c108446a8 100644
--- a/hw/xen/xen_pt_stub.c
+++ b/hw/xen/xen_pt_stub.c
@@ -20,3 +20,7 @@ void xen_igd_gfx_pt_set(bool value, Error **errp)
         error_setg(errp, "Xen PCI passthrough support not built in");
     }
 }
+
+void xen_igd_reserve_slot(PCIBus *pci_bus)
+{
+}
-- 
2.37.2