On 05/10/2026 20:19, Laine Stump wrote:
When a new domain is created and no busNr is given, the default value
decided on by libvirt at that time is written into the XML, so
subsequent starts of that particular domain using that definition will
use the same busNr - only newly created domains will get a different
default busNr than they used to. So in that case, this change in default
setting has 0 effect.
There are management stacks that re-create the XML for every power on,
so every power cycle these VMs would see different SBDFs across the
libvirt upgrade and some VMs (e.g., Windows) get upset if SBDFs change
(of which I'm sure you're aware).
If you think management stacks that do that should also take care of the
busNr allocations then I personally find that acceptable (as the
management stack I care about already does this), but I'm not sure I'd
want to make assumptions about the behaviour of every one.
So whilst I agree that the current bus number allocation algorithm isn't
very useful, I do think a new algorithm needs to be explicitly requested
by the user or management stack.
You could have something like:
<controller type='pci' index='1' model='pcie-expander-bus'>
<model name='pxb-pcie'/>
<target busNr='+16'/>
...
Where the +16 means reserve 16 "spare" bus numbers for downstream
devices/ports, without having to care about exactly which bus numbers
are allocated. The default value is the current behaviour of "+2".
If you wish to continue with the patch as-is, I think the description
could benefit from a clearer description of the current allocation
algorithm (e.g., clarify that numbering starts from the highest bus and
goes down).
David
