On Thu, Sep 11, 2025 at 2:18 PM Beñat Gartzia Arruabarrena <bgart...@redhat.com> wrote: > > Hello, > > I am trying to find a solution to an issue related to vhost-vdpa devices > and random mac addresses being assigned to them, if not set by the user. > > vDPA devices are assigned a MAC address the time they are created. That > happens before creating the VM with qemu or libvirt. If the mac address > of the vdpa device in the qemu command line does not match the mac > address that was assigned to the vdpa device at creation time, the > network traffic will not work.
Note that MAC address is not the only thing that needs to be worried here: 1) MTU 2) queue pairs 3) host_features etc > > If the user does not explicitly configure a mac address for the vdpa > device in the domain xml, libvirt will generate a random one and use it > instead in the qemu command line. This leads to an interface that does > not work. > > If I understood correctly, qemu is not able to update the mac address of > the vdpa device based on the command line configuration, so setting the > mac address generated by libvirt is not an option. Currently, a patch > that introduces a set of checks comparing cmdline and device mac > addresses is under review: > > > https://lore.kernel.org/qemu-devel/20250417102522.4125379-1-l...@redhat.com/ > > The implementation also adds the logic to configure the vdpa device mac > address in the virtio net device if a mac address was not provided in > the command line. > > I was trying to write a patch such that it would allow vdpa devices to > have an empty mac address in the domain xml as an exception. The draft > allowed such case in virDomainNetDefParseXML and virDomainNetDefFormat > and avoided writing an empty mac address in the qemu command line by > modifying the logic in qemuBuildNicDevProps. However, there are plenty > of other functions and places that would expect the mac address to be > there. Guarding against the empty mac addr case in all those places, is > not desirable, I think. > > I'm not sure if libvirt could read the mac address of the device being > assigned to the VM instead of generating a random mac address in the > case of vdpa devices. It could probably require some code from qemu > being duplicated. Could that be an acceptable alternative? I am also > looking for any other recommendation on how I could look into this > issue. Or maybe introduce an option to not specify any guest visible attributes via qemu cli. Thanks > > Thanks a lot. > > Regards, > Beñat >
