KVM: arm/arm64: Don't invoke defacto-CnP on first run

When KVM finds itself switching between two vCPUs of the same VM
on one physical CPU it has to invalidate the TLB for this VMID
to avoid unintended sharing of TLB entries between vCPU.

This is done by tracking the 'last_vcpu_ran' as a percpu variable
for each vm.

kvm_arch_init_vm() is careful to initialise these to an impossible
vcpu id, but we never check for this. The first time
vm_arch_vcpu_load() is called on a new physical CPU, we will fail
the last_ran check and invalidate the TLB.

Now that we have an errata workaround in this path, it means we
trigger the workaround whenever a guest is migrated to a new CPU.

Check for the impossible vcpu id, and skip defacto-CnP.

Change-Id: Id2b1fb291ab925a59de2b8fd0e29892898eb4d20
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Patch-mainline: linux-arm-kernel @ 11/14/19, 14:59
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>