[makomk]

The mask of flags that the guest isn't allowed to set is created by taking a whitelist of flags that it is allowed to set and inverting it, so that every flag that's not explicitly allowed is denied. They're flags that the guest was intentionally granted the ability to change. The problem is that Xen subtly mishandled the consequences of a guest changing them in a way that wouldn't affect normal guests. I haven't seen a solution to this that doesn't, in the end, basically boil down to not making that mistake in the first place.