It’s a QEMU regression bug (bad commit: c6beefd674ff). QEMU only saves acked
features for one vhost-net when vhost quits. When vhost reconnects to
virtio-net/virtio-pmd in multi-queue situations, the features been set multiple
times are not consistent. QEMU-5.2.0 fixes this issue in commit f66337bdbfda
(“vhost-user: save features of multiqueues if chardev is closed”).

Implication
Vhost cannot reconnect back to virtio-net/virtio-pmd normally.

Resolution/Workaround:
It is possible to filter the incorrect acked features at vhost-user side.

Affected Environment/Platform:
ALL.

Driver/Module:
Virtual Device Poll Mode Driver (PMD).