1 files changed, 53 insertions, 3 deletions

diff --git a/hw/pci-host/gpex.c b/hw/pci-host/gpex.c
index 2bdbe7b456..a6752fac5e 100644
--- a/hw/pci-host/gpex.c
+++ b/hw/pci-host/gpex.c
@@ -83,12 +83,51 @@ static void gpex_host_realize(DeviceState *dev, Error **errp)
     int i;
 
     pcie_host_mmcfg_init(pex, PCIE_MMCFG_SIZE_MAX);
+    sysbus_init_mmio(sbd, &pex->mmio);
+
+    /*
+     * Note that the MemoryRegions io_mmio and io_ioport that we pass
+     * to pci_register_root_bus() are not the same as the
+     * MemoryRegions io_mmio_window and io_ioport_window that we
+     * expose as SysBus MRs. The difference is in the behaviour of
+     * accesses to addresses where no PCI device has been mapped.
+     *
+     * io_mmio and io_ioport are the underlying PCI view of the PCI
+     * address space, and when a PCI device does a bus master access
+     * to a bad address this is reported back to it as a transaction
+     * failure.
+     *
+     * io_mmio_window and io_ioport_window implement "unmapped
+     * addresses read as -1 and ignore writes"; this is traditional
+     * x86 PC behaviour, which is not mandated by the PCI spec proper
+     * but expected by much PCI-using guest software, including Linux.
+     *
+     * In the interests of not being unnecessarily surprising, we
+     * implement it in the gpex PCI host controller, by providing the
+     * _window MRs, which are containers with io ops that implement
+     * the 'background' behaviour and which hold the real PCI MRs as
+     * subregions.
+     */
     memory_region_init(&s->io_mmio, OBJECT(s), "gpex_mmio", UINT64_MAX);
     memory_region_init(&s->io_ioport, OBJECT(s), "gpex_ioport", 64 * 1024);
 
-    sysbus_init_mmio(sbd, &pex->mmio);
-    sysbus_init_mmio(sbd, &s->io_mmio);
-    sysbus_init_mmio(sbd, &s->io_ioport);
+    if (s->allow_unmapped_accesses) {
+        memory_region_init_io(&s->io_mmio_window, OBJECT(s),
+                              &unassigned_io_ops, OBJECT(s),
+                              "gpex_mmio_window", UINT64_MAX);
+        memory_region_init_io(&s->io_ioport_window, OBJECT(s),
+                              &unassigned_io_ops, OBJECT(s),
+                              "gpex_ioport_window", 64 * 1024);
+
+        memory_region_add_subregion(&s->io_mmio_window, 0, &s->io_mmio);
+        memory_region_add_subregion(&s->io_ioport_window, 0, &s->io_ioport);
+        sysbus_init_mmio(sbd, &s->io_mmio_window);
+        sysbus_init_mmio(sbd, &s->io_ioport_window);
+    } else {
+        sysbus_init_mmio(sbd, &s->io_mmio);
+        sysbus_init_mmio(sbd, &s->io_ioport);
+    }
+
     for (i = 0; i < GPEX_NUM_IRQS; i++) {
         sysbus_init_irq(sbd, &s->irq[i]);
         s->irq_num[i] = -1;
@@ -108,6 +147,16 @@ static const char *gpex_host_root_bus_path(PCIHostState *host_bridge,
     return "0000:00";
 }
 
+static Property gpex_host_properties[] = {
+    /*
+     * Permit CPU accesses to unmapped areas of the PIO and MMIO windows
+     * (discarding writes and returning -1 for reads) rather than aborting.
+     */
+    DEFINE_PROP_BOOL("allow-unmapped-accesses", GPEXHost,
+                     allow_unmapped_accesses, true),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
 static void gpex_host_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -117,6 +166,7 @@ static void gpex_host_class_init(ObjectClass *klass, void *data)
     dc->realize = gpex_host_realize;
     set_bit(DEVICE_CATEGORY_BRIDGE, dc->categories);
     dc->fw_name = "pci";
+    device_class_set_props(dc, gpex_host_properties);
 }
 
 static void gpex_host_initfn(Object *obj)