On Mon, Oct 31, 2022 at 6:55 PM Andrea Bolognani <abolo...@redhat.com> wrote:
> On Mon, Oct 31, 2022 at 04:32:27PM +0200, Edward Haas wrote: > > 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. > > The machine type might not change from q35 to i440fx and vice versa, > but since the domain XML is constructed every time a KubeVirt VM is > started, the machine type might be q35-6.0 on one boot and q35-7.0 > the next one if a KubeVirt upgrade that comes with a new version of > QEMU has happened in between. > > This is unlikely to make a difference in terms of PCI addresses seen > in the guest OS, but it's still not accurate to say that the machine > type will not change. > Thank you for the clarification. It makes me wonder now what are the actual implications of the machine type change. > Live migration is a separate matter, as the machine type will > definitely not change while the VM is running. > > > 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? > > A change in libvirt (again, due to a KubeVirt upgrade in between two > boots of the same VM) might result in different PCI addresses being > assigned to devices despite the same input XML. > > We generally try fairly hard to avoid this kind of situation, but we > can only really guarantee stable PCI addresses for the lifetime of a > VM that has been defined and can't promise that the same input XML > will result in the same guest ABI when using different versions of > libvirt. > I would expect the PCI addresses that have been explicitly set in the domxml [2] to be honored. We cannot assume that? I mainly referred to that input option, not to the expectation that the generated configuration (of the domxml) to be identical between different versions. [2] https://libvirt.org/formatdomain.html#device-addresses > > -- > Andrea Bolognani / Red Hat / Virtualization > >
