xen: enable memory and I/O decoding before passing PCI device to guest

Otherwise, with v1 of the XSA-126 fix in place, nothing will do this,
leaving the device unusable except for malicious purposes.

This is part of CVE-2015-2756 / XSA-126.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

--- a/hw/pass-through.c
+++ b/hw/pass-through.c
@@ -1905,7 +1905,7 @@ static int pt_dev_is_virtfn(struct pci_d
     return rc;
 }
 
-static int pt_register_regions(struct pt_dev *assigned_device)
+static int pt_register_regions(struct pt_dev *assigned_device, uint16_t *cmd)
 {
     int i = 0;
     uint32_t bar_data = 0;
@@ -1925,17 +1925,26 @@ static int pt_register_regions(struct pt
 
             /* Register current region */
             if ( pci_dev->base_addr[i] & PCI_ADDRESS_SPACE_IO )
+            {
                 pci_register_io_region((PCIDevice *)assigned_device, i,
                     (uint32_t)pci_dev->size[i], PCI_ADDRESS_SPACE_IO,
                     pt_ioport_map);
+                *cmd |= PCI_COMMAND_IO;
+            }
             else if ( pci_dev->base_addr[i] & PCI_ADDRESS_SPACE_MEM_PREFETCH )
+            {
                 pci_register_io_region((PCIDevice *)assigned_device, i,
                     (uint32_t)pci_dev->size[i], PCI_ADDRESS_SPACE_MEM_PREFETCH,
                     pt_iomem_map);
+                *cmd |= PCI_COMMAND_MEMORY;
+            }
             else
+            {
                 pci_register_io_region((PCIDevice *)assigned_device, i,
                     (uint32_t)pci_dev->size[i], PCI_ADDRESS_SPACE_MEM,
                     pt_iomem_map);
+                *cmd |= PCI_COMMAND_MEMORY;
+            }
 
             PT_LOG("IO region registered (size=0x%08x base_addr=0x%08x)\n",
                 (uint32_t)(pci_dev->size[i]),
@@ -4211,6 +4220,7 @@ static struct pt_dev * register_real_dev
     struct pt_dev *assigned_device = NULL;
     struct pci_dev *pci_dev;
     uint8_t e_device, e_intx;
+    uint16_t cmd = 0;
     char *key, *val;
     int msi_translate, power_mgmt;
 
@@ -4300,7 +4310,7 @@ static struct pt_dev * register_real_dev
         assigned_device->dev.config[i] = pci_read_byte(pci_dev, i);
 
     /* Handle real device's MMIO/PIO BARs */
-    pt_register_regions(assigned_device);
+    pt_register_regions(assigned_device, &cmd);
 
     /* Setup VGA bios for passthroughed gfx */
     if ( setup_vga_pt(assigned_device) < 0 )
@@ -4378,6 +4388,10 @@ static struct pt_dev * register_real_dev
     }
 
 out:
+    if (cmd)
+        pci_write_word(pci_dev, PCI_COMMAND,
+            *(uint16_t *)(&assigned_device->dev.config[PCI_COMMAND]) | cmd);
+
     PT_LOG("Real physical device %02x:%02x.%x registered successfuly!\n"
            "IRQ type = %s\n", r_bus, r_dev, r_func,
            assigned_device->msi_trans_en? "MSI-INTx":"INTx");