[Qemu-devel] [PATCH repost] pci: fix migration device path for devices behind nested bridges

["Michael S. Tsirkin" <mst@redhat.com>]

We were using bus number in the device path, which is clearly
broken as this number is guest-assigned for all devices
except the root.

Replace this by a hierarchical list of slot/function numbers, walking
the path from root down to device. Add :00 after the domain number
so that if there are no nested bridges, this is compatible
with what we have now.

Note: as pointed out by Gleb, using openfirmware paths
might be cleaner, doing this would break compatibility though,
and the IDs used are not guest or user visible at all,
so it probably doesn't matter what they are exactly.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

OK I think I'll just apply the below, it seems better for the
next release than the alternative of disabling migration
completely, and the IDs used are completely transparent to
the user, so it probably doesn't matter what they are exactly.

 hw/pci.c |   44 ++++++++++++++++++++++++++++++++++++--------
 1 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/hw/pci.c b/hw/pci.c
index 8f6fcf8..aed2d42 100644
--- a/hw/pci.c
+++ b/hw/pci.c
@@ -1826,15 +1826,41 @@ static void pcibus_dev_print(Monitor *mon, DeviceState *dev, int indent)
 
 static char *pcibus_get_dev_path(DeviceState *dev)
 {
-    PCIDevice *d = (PCIDevice *)dev;
-    char path[16];
-
-    snprintf(path, sizeof(path), "%04x:%02x:%02x.%x",
-             pci_find_domain(d->bus),
-             0 /* TODO: need a persistent path for nested buses.
-                * Note: pci_bus_num(d->bus) is not right as it's guest
-                * assigned. */,
-             PCI_SLOT(d->devfn), PCI_FUNC(d->devfn));
-
-    return strdup(path);
+    PCIDevice *d = container_of(dev, PCIDevice, qdev);
+    PCIDevice *t;
+    int slot_depth;
+    /* Path format: Domain:00:Slot.Function:Slot.Function....:Slot.Function.
+     * 00 is added here to make this format compatible with
+     * domain:Bus:Slot.Func for systems without nested PCI bridges.
+     * Slot.Function list specifies the slot and function numbers for all
+     * devices on the path from root to the specific device. */
+    int domain_len = strlen("DDDD:00");
+    int slot_len = strlen(":SS.F");
+    int path_len;
+    char *path, *p;
+
+    /* Calculate # of slots on path between device and root. */;
+    slot_depth = 0;
+    for (t = d; t; t = t->bus->parent_dev)
+        ++slot_depth;
+
+    path_len = domain_len + slot_len * slot_depth;
+
+    /* Allocate memory, fill in the terminating null byte. */
+    path = malloc(path_len + 1 /* For '\0' */);
+    path[path_len] = '\0';
+
+    /* First field is the domain. */
+    snprintf(path, domain_len, "%04x:00", pci_find_domain(d->bus));
+
+    /* Fill in slot numbers. We walk up from device to root, so need to print
+     * them in the reverse order, last to first. */
+    p = path + path_len;
+    for (t = d; t; t = t->bus->parent_dev) {
+        p -= slot_len;
+        snprintf(p, slot_len, ":%02x.%x", PCI_SLOT(t->devfn), PCI_FUNC(d->devfn));
+    }
+
+    return path;
 }
 
-- 
1.7.3.2.91.g446ac