Le 04/02/2026 à 10:02, Daniel P. Berrangé a écrit :
On Tue, Feb 03, 2026 at 11:12:18PM +0100, Bertrand Leconte wrote:
Hello,
I am using libvirt within Qubes OS (https://www.qubes-or.org).
My laptop is a Lenovo P16 Gen 3 with an Intel Core Ultra 9 275HX, which is
in the Arrow Lake family. The associated PCH is an Intel 800 Series WM880.
The particularity of this platform is that the PCH includes a dedicated PCI
Root Complex in addition to the CPU one.
That means that I have devices connected to `0000:00` bus (addresses like
0000:00:xx.x) and devices connected to `0000:80` bus (addresses like
0000:80:xx.x).
The consequence of that architecture is that there is no PCI Root Port
serving the `0000:80` bus.
When I want to PCI passthrough my USB controller device `0000:80:14.0`, the
function `virPCIDeviceIsBehindSwitchLackingACS()` says that `Failed to find
parent device for 0000:80:14.0`.
When I look into the source code, it looks like it enumerates all PCI
devices, searching for the Root Port managing the bus `0000:80`. As I said
before, it cannot find any as it's A Root Complex that manage the `0000:80`
bus and not a Root Port (a Root Complex it's not a PCI/PCIe device).
However, for any device connected to PCI bus `0000:00`, it cannot find any
PCI Root Port either but there is an exception in the code for it.
@Bloged (one of Qubes OS users) proposed in
https://forum.qubes-os.org/t/aistone-x6ar57ty-aka-tongfang-x6-16-intel-high-end-performance/37944/2
to change the code to
diff --git a/src/util/virpci.c b/src/util/virpci.c
index 3816369..fbed8f5 100644
--- a/src/util/virpci.c
+++ b/src/util/virpci.c
@@ -2555,12 +2555,20 @@ virPCIDeviceIsBehindSwitchLackingACS(virPCIDevice
*dev)
if (virPCIDeviceGetParent(dev, &parent) < 0)
return -1;
if (!parent) {
- /* if we have no parent, and this is the root bus, ACS doesn't come
- * into play since devices on the root bus can't P2P without going
- * through the root IOMMU.
+ /* if we have no parent, and this is a root-like bus (0
or 0x80),
+ * ACS doesn't come into play since devices on these buses can't
P2P
+ * without going through the root IOMMU.
+ *
+ * Arrow Lake (Intel Core Ultra 200 series) uses bus 0x80 for
integrated
+ * devices like USB controllers, with a complex PCH topology that
confuses
+ * parent device detection. Treat bus 0x80 like bus 0 for ACS
purposes.
*/
if (dev->address.bus == 0) {
return 0;
+ } else if (dev->address.bus == 0x80) {
+ VIR_DEBUG("%s %s: Arrow Lake bus 0x80 device, treating as root
bus for ACS check",
+ dev->id, dev->name);
+ return 0;
} else {
virReportError(VIR_ERR_INTERNAL_ERROR,
_("Failed to find parent device for %1$s"),
--
This patch works, but it does not look very generic.
Indeed, the correct way would be to look at /sys/devices/pciNNNN.NN to
identity root complexes, but.....
In addition to that, a thread in 2017 (this mailing list) seems to indicate
that this part of the code may not be necessary:
https://lists.libvirt.org/archives/list/[email protected]/thread/QRI4L5YFGBAV7NCTJ5OOJ7GBYGHQ7WZQ/#K5WRR6WTAUEPRQBWJVJSMX554DTYFTYH.
My question: can someone that knows this part of the code have a look to see
what is the best way to correct it: either propose a patch to
`virPCIDeviceIsBehindSwitchLackingACS` that is more generic or to change the
rest of the code so it's no more used?
....That thread certainly suggests we have a large set of code that can
be purged. Libvirt only needs to support the 2 most recent releases of
each distro, and since the kerenl removed legacy device assignment 8
years ago I see no reason for us to keep this.
This still begs the question of how you hit this code path to begin
with, since Laine's message above suggests it ought to be impossible
to hit already.
I asked the question under the issue I created on Qubes' github
(https://github.com/QubesOS/qubes-issues/issues/10393). I hope someone
there will be able to provide a hint to understand.
I will also look at it, but I am just a Qubes user, so I am not sure to
understand what it's done that way (and it will take a lot of time)
Thanks,
Bertrand
With regards,
Daniel
