Re: Predictable and consistent net interface naming in guests

On 11/1/22 7:46 AM, Igor Mammedov wrote: > On Mon, 31 Oct 2022 14:48:54 +0000 > Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > >> On Mon, Oct 31, 2022 at 04:32:27PM +0200, Edward Haas wrote: >>> Hi Igor and Laine, >>> >>> I would like to revive a 2 years old discussion [1] about consistent network >>> interfaces in the guest. >>> >>> That discussion mentioned that a guest PCI address may change in two cases: >>> - The PCI topology changes. >>> - The machine type changes. >>> >>> Usually, the machine type is not expected to change, especially if one >>> wants to allow migrations between nodes. >>> I would hope to argue this should not be problematic in practice, because >>> guest images would be made per a specific machine type. >>> >>> Regarding the PCI topology, I am not sure I understand what changes >>> need to occur to the domxml for a defined guest PCI address to change. >>> The only think that I can think of is a scenario where hotplug/unplug is >>> used, >>> but even then I would expect existing devices to preserve their PCI address >>> and the plug/unplug device to have a reserved address managed by the one >>> acting on it (the management system). >>> >>> Could you please help clarify in which scenarios the PCI topology can cause >>> a mess to the naming of interfaces in the guest? >>> >>> Are there any plans to add the acpi_index support? >> >> This was implemented a year & a half ago >> >> https://libvirt.org/formatdomain.html#network-interfaces >> >> though due to QEMU limitations this only works for the old >> i440fx chipset, not Q35 yet. > > Q35 should work partially too. In its case acpi-index support > is limited to hotplug enabled root-ports and PCIe-PCI bridges. > One also has to enable ACPI PCI hotplug (it's enled by default > on recent machine types) for it to work (i.e.it's not supported > in native PCIe hotplug mode). > > So if mgmt can put nics on root-ports/bridges, then acpi-index > should just work on Q35 as well. With only a few exceptions (e.g. the first ich9 audio device, which is placed directly on the root bus at 00:1B.0 because that is where the ich9 audio device is located on actual Q35 hardware), libvirt will automatically put all PCI devices (including network interfaces) on a pcie-root-port. After seeing reports that "acpi index doesn't work with Q35 machinetypes" I just assumed that was correct and didn't try it. But after seeing the "should work partially" statement above, I tried it just now and an <interface> of a Q35 guest that had its PCI address auto-assigned by libvirt (and so was placed on a pcie-root-port)m and had <acpi index='4'/> was given the name "eno4". So what exactly is it that *doesn't* work?