If you are running your system with SMT enabled, you need to add the following lines to /etc/default/grub
GRUB_CMDLINE_XEN_DEFAULT="$GRUB_CMDLINE_XEN_DEFAULT smt=off"
GRUB_CMDLINE_XEN_DEFAULT="$GRUB_CMDLINE_XEN_DEFAULT smt=on"
It’s the only way to keep kernel updates from disabling smt, by automatically adding the smt=off line to the end of the file.