ARM never properly implemented grant table v2, either in the hypervisor or in Linux. Unfortunately, an ARM guest can still request v2 grant tables; they will simply not be properly set up, resulting in subsequent grant-related hypercalls hitting BUG() checks.
An unprivileged guest can cause a BUG() check in the hypervisor, resulting in a denial-of-service.
Only ARM systems are vulnerable. All supported versions of Xen are vulnerable.