In function virPCIDeviceIsBehindSwitchLackingACS, I noticed that(line 8): 1 if (virPCIDeviceGetParent(dev, &parent) < 0) 2 return -1; 3 if (!parent) { 4 /* if we have no parent, and this is the root bus, ACS doesn't come 5 * into play since devices on the root bus can't P2P without going 6 * through the root IOMMU. 7 */ 8 if (dev->address.bus == 0) { 9 return 0; 10 } else { 11 virReportError(VIR_ERR_INTERNAL_ERROR, 12 _("Failed to find parent device for %s"), 13 dev->name); 14 return -1; 15 } 16 } Why we just return 0 only if device's bus is 0? In my server, I can see a root bus which bus number is greater than 0, see the results(just a part) after I run lspci -t: +-[0000:80]-+-02.0-[81-83]--+-00.0 | | \-00.1 | +-05.0 | +-05.1 | +-05.2 | \-05.4 +-[0000:7f]-+-08.0 | +-08.2 | +-08.3 | + . . . | \-1f.2 \-[0000:00]-+-00.0 +-01.0-[01]----00.0 +-02.0-[02]--+-00.0 | +-00.1 | +-00.2 | \-00.3 +-02.2-[03]-- +-03.0-[04-0b]----00.0-[05-0b]--+-08.0-[06-08]----00.0 | \-10.0-[09-0b]----00.0 +-05.0 +-05.1 +-05.2 +-05.4 +-11.0 +-11.4 +-16.0 +-16.1 +-1a.0 If I assign the device 0000:81:00.0 to a VM, I get "Failed to find parent device". I think I should get no error with return value 0 just like bus number is 0, because bus 80 is the root bus as well in my case. In the <<Intel C610 Series Chipset and Intel X99 Chipset Platform Controller Hub(PCH)>> Datasheet, I found that(Chapter 9.1): For some server platforms, it may be desirable to have multiple PCHs in the system Which means some PCH's may reside on a bus greater than 0. So, is this a bug? Thanks, Zongyong Wu