libvirt users - Nov 2022 - Predictable and consistent net interface naming in guests

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.


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

Thank you both for the quick response.

On Mon, Oct 31, 2022 at 4:49 PM Daniel P. Berrang? <berrange at
redhat.com>
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.
>
I think most deployments today use Q35.
Are there plans to resolve it there?

BTW, should this limitation be added to the documentation?

>
> With regards,
> Daniel
> --
> |: https://berrange.com      -o-
> https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-
> https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-
> https://www.instagram.com/dberrange :|
>
>
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On Mon, 31 Oct 2022 14:48:54 +0000
Daniel P. Berrang? <berrange at redhat.com> 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 regards,
> Daniel