[Qemu-devel] Re: [PATCH] PCI: Bus number from the bridge, not the device

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

On Mon, Nov 08, 2010 at 07:52:12AM -0700, Alex Williamson wrote:
> On Mon, 2010-11-08 at 13:22 +0200, Michael S. Tsirkin wrote:
> > On Mon, Oct 04, 2010 at 03:53:11PM -0600, Alex Williamson wrote:
> > > pcibus_dev_print() was erroneously retrieving the device bus
> > > number from the secondary bus number offset of the device
> > > instead of the bridge above the device.  This ends of landing
> > > in the 2nd byte of the 3rd BAR for devices, which thankfully
> > > is usually zero.  pcibus_get_dev_path() copied this code,
> > > inheriting the same bug.  pcibus_get_dev_path() is used for
> > > ramblock naming, so changing it can effect migration.  However,
> > > I've only seen this byte be non-zero for an assigned device,
> > > which can't migrate anyway, so hopefully we won't run into
> > > any issues.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > 
> > Good catch. Applied.
> > I don't really see why do we put the dev path
> > in the bus object: why not let device supply its name?
> 
> Because the device name is not unique.  This came about from the
> discussion about how to create a canonical device path that Gleb and
> Markus are again trying to hash out.  If we go up to the bus and get the
> bus address, we have a VM unique name.  Unfortunately, it's difficult to
> define what the bus should print in all cases (ISA), but since they
> don't do hotplug and typically don't allocate ramblocks, we can mostly
> ignore it for this use case.
> 
> > And I think this will affect nested bridges. However they are currently
> > broken anyway: we really must convert to topological names as bus number
> > is guest-assigned - they don't have to be unique, even.
> 
> Yes, nested bridges are a problem.  How can the seg/bus/devfn not be
> unique?

Bus numbers for nested bridges are guest assigned. We start with 0 after reset.

> > What does fixing this involve? Just changing pcibus_get_dev_path?
> 
> How do you plan to fix it?  Don't forget that migration depends on these
> names, so some kind of compatibility layer would be required.  Thanks,
> 
> Alex

Replace bus number with slot numbers of parent bridges up to the root.
This works for root bridge in a compatible way because bus number there
is hard-coded to 0.
IMO nested bridges are broken anyway, no way to be compatible there.


Gleb, Markus, I think the following should be sufficient for PCI.  What
do you think?  Also - do we need to update QMP/monitor to teach them to
work with these paths?

This is on top of Alex's patch, completely untested.


pci: fix device path for devices behind nested bridges

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.

Fix by using hierarchical list of slots, walking the path
from root down to device, instead. Add :00 as bus number
so that if there are no nested bridges, this is compatible
with what we have now.

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

diff --git a/hw/pci.c b/hw/pci.c
index 7d12473..fa98d94 100644
--- a/hw/pci.c
+++ b/hw/pci.c
@@ -1826,13 +1826,45 @@ 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), pci_bus_num(d->bus),
-             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:Slot....:Slot.Function.
+     * 00 is added here to make this format compatible with
+     * domain:Bus:Slot.Func for systems without nested PCI bridges.
+     * Slot list specifies the slot numbers for all devices on the
+     * path from root to the specific device. */
+    int domain_len = strlen("DDDD:00");
+    int func_len = strlen(".F");
+    int slot_len = strlen(":SS");
+    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 + bus_len + slot_len * slot_depth + func_len;
+
+    /* 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", pci_find_domain(d->bus));
+
+    /* Leave space for slot numbers and fill in function number. */
+    p = path + domain_len + slot_len * slot_depth;
+    snprintf(p, func_len, ".%02x", PCI_FUNC(d->devfn));
+
+    /* Fill in slot numbers. We walk up from device to root, so need to print
+     * them in the reverse order, last to first. */
+    for (t = d; t; t = t->bus->parent_dev) {
+        p -= slot_len;
+        snprintf(p, slot_len, ":%x", PCI_SLOT(t->devfn));
+    }
+
+    return path;
 }