The issue relates to the context switch logic within Xen. Specifically, Xen attempts to skip an IBPB (Instruction Barrier Page Base) when a virtual CPU (vCPU) returns to a CPU it previously ran on. While this action maintains isolation between vCPUs from Xen's perspective, it prevents the guest kernel from correctly isolating between tasks. This can lead to a scenario where a new task is executed with the training data of a previous task still present in the Branch Target Buffer (BTB).