A particularly annoying userspace could create a vCPU after KVM has
computed mpidr_data for the VM, either by racing against VGIC
initialization or having a userspace irqchip.

In any case, this means mpidr_data no longer fully describes the VM, and
attempts to find the new vCPU with kvm_mpidr_to_vcpu() will fail. The
fix is to discard mpidr_data altogether, as it is only a performance
optimization and not required for correctness. In all likelihood KVM
will recompute the mappings when KVM_RUN is called on the new vCPU.

Note that reads of mpidr_data are not guarded by a lock; promote to RCU
to cope with the possibility of mpidr_data being invalidated at runtime.

Fixes: 54a8006d ("KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is available")
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240508071952.2035422-1-oliver.upton@linux.devSigned-off-by: Marc Zyngier <maz@kernel.org>