The BDSM register is mirrored into MMIO space at least for gen 11 and
later devices. Unfortunately, the Windows driver reads the register
value from MMIO space instead of PCI config space for those devices [1].
Therefore, we either have to keep a 1:1 mapping for the host and guest
address or we have to emulate the MMIO register too. Using the igd in
legacy mode is already hard due to it's many constraints. Keeping a 1:1
mapping may not work in all cases and makes it even harder to use. An
MMIO emulation has to trap the whole MMIO page. This makes accesses to
this page slower compared to using second level address translation.
Nevertheless, it doesn't have any constraints and I haven't noticed any
performance degradation yet making it a better solution.

[1] https://github.com/projectacrn/acrn-hypervisor/blob/5c351bee0f6ae46250eefc07f44b4a31e770f3cf/devicemodel/hw/pci/passthrough.c#L650-L653

Signed-off-by: Corvin Köhne <c.koehne@beckhoff.com>
---
v2:
	* omit unnecessary leXX_to_cpu calls
	* make use of IGD_BDSM_MMIO_OFFSET define

 hw/vfio/igd.c        | 98 ++++++++++++++++++++++++++++++++++++++++++++
 hw/vfio/pci-quirks.c |  1 +
 hw/vfio/pci.h        |  1 +
 3 files changed, 100 insertions(+)

diff --git a/hw/vfio/igd.c b/hw/vfio/igd.c
index 0b6533bbf7..0d68c6a451 100644
--- a/hw/vfio/igd.c
+++ b/hw/vfio/igd.c
@@ -374,6 +374,104 @@ static const MemoryRegionOps vfio_igd_index_quirk = {
     .endianness = DEVICE_LITTLE_ENDIAN,
 };
 
+#define IGD_BDSM_MMIO_OFFSET 0x1080C0
+
+static uint64_t vfio_igd_quirk_bdsm_read(void *opaque,
+                                          hwaddr addr, unsigned size)
+{
+    VFIOPCIDevice *vdev = opaque;
+    uint64_t offset;
+
+    offset = IGD_BDSM_GEN11 + addr;
+
+    switch (size) {
+    case 1:
+        return pci_get_byte(vdev->pdev.config + offset);
+    case 2:
+        return pci_get_word(vdev->pdev.config + offset);
+    case 4:
+        return pci_get_long(vdev->pdev.config + offset);
+    case 8:
+        return pci_get_quad(vdev->pdev.config + offset);
+    default:
+        hw_error("igd: unsupported read size, %u bytes", size);
+        break;
+    }
+
+    return 0;
+}
+
+static void vfio_igd_quirk_bdsm_write(void *opaque, hwaddr addr,
+                                       uint64_t data, unsigned size)
+{
+    VFIOPCIDevice *vdev = opaque;
+    uint64_t offset;
+
+    offset = IGD_BDSM_GEN11 + addr;
+
+    switch (size) {
+    case 1:
+        pci_set_byte(vdev->pdev.config + offset, data);
+        break;
+    case 2:
+        pci_set_word(vdev->pdev.config + offset, data);
+        break;
+    case 4:
+        pci_set_long(vdev->pdev.config + offset, data);
+        break;
+    case 8:
+        pci_set_quad(vdev->pdev.config + offset, data);
+        break;
+    default:
+        hw_error("igd: unsupported read size, %u bytes", size);
+        break;
+    }
+}
+
+static const MemoryRegionOps vfio_igd_bdsm_quirk = {
+    .read = vfio_igd_quirk_bdsm_read,
+    .write = vfio_igd_quirk_bdsm_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+};
+
+void vfio_probe_igd_bar0_quirk(VFIOPCIDevice *vdev, int nr)
+{
+    VFIOQuirk *quirk;
+    int gen;
+
+    /*
+     * This must be an Intel VGA device at address 00:02.0 for us to even
+     * consider enabling legacy mode. Some driver have dependencies on the PCI
+     * bus address.
+     */
+    if (!vfio_pci_is(vdev, PCI_VENDOR_ID_INTEL, PCI_ANY_ID) ||
+        !vfio_is_vga(vdev) || nr != 0 ||
+        &vdev->pdev != pci_find_device(pci_device_root_bus(&vdev->pdev),
+                                       0, PCI_DEVFN(0x2, 0))) {
+        return;
+    }
+
+    /*
+     * Only on IGD devices of gen 11 and above, the BDSM register is mirrored
+     * into MMIO space and read from MMIO space by the Windows driver.
+     */
+    gen = igd_gen(vdev);
+    if (gen < 11) {
+        return;
+    }
+
+    quirk = vfio_quirk_alloc(1);
+    quirk->data = vdev;
+
+    memory_region_init_io(&quirk->mem[0], OBJECT(vdev), &vfio_igd_bdsm_quirk,
+                          vdev, "vfio-igd-bdsm-quirk", 8);
+    memory_region_add_subregion_overlap(vdev->bars[0].region.mem,
+                                        IGD_BDSM_MMIO_OFFSET, &quirk->mem[0],
+                                        1);
+
+    QLIST_INSERT_HEAD(&vdev->bars[nr].quirks, quirk, next);
+}
+
 void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr)
 {
     g_autofree struct vfio_region_info *rom = NULL;
diff --git a/hw/vfio/pci-quirks.c b/hw/vfio/pci-quirks.c
index 39dae72497..d37f722cce 100644
--- a/hw/vfio/pci-quirks.c
+++ b/hw/vfio/pci-quirks.c
@@ -1259,6 +1259,7 @@ void vfio_bar_quirk_setup(VFIOPCIDevice *vdev, int nr)
     vfio_probe_nvidia_bar0_quirk(vdev, nr);
     vfio_probe_rtl8168_bar2_quirk(vdev, nr);
 #ifdef CONFIG_VFIO_IGD
+    vfio_probe_igd_bar0_quirk(vdev, nr);
     vfio_probe_igd_bar4_quirk(vdev, nr);
 #endif
 }
diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
index bf67df2fbc..5ad090a229 100644
--- a/hw/vfio/pci.h
+++ b/hw/vfio/pci.h
@@ -215,6 +215,7 @@ void vfio_setup_resetfn_quirk(VFIOPCIDevice *vdev);
 bool vfio_add_virt_caps(VFIOPCIDevice *vdev, Error **errp);
 void vfio_quirk_reset(VFIOPCIDevice *vdev);
 VFIOQuirk *vfio_quirk_alloc(int nr_mem);
+void vfio_probe_igd_bar0_quirk(VFIOPCIDevice *vdev, int nr);
 void vfio_probe_igd_bar4_quirk(VFIOPCIDevice *vdev, int nr);
 
 extern const PropertyInfo qdev_prop_nv_gpudirect_clique;
-- 
2.46.0

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