KVM: PPC: Book3S HV: Optimise mmio emulation for devices on FAST_MMIO_BUS

Devices on the KVM_FAST_MMIO_BUS by definition have length zero and are
thus used for notification purposes rather than data transfer. For
example eventfd for virtio devices.

This means that when emulating mmio instructions which target devices on
this bus we can immediately handle them and return without needing to load
the instruction from guest memory.

For now we restrict this to stores as this is the only use case at
present.

For a normal guest the effect is negligible, however for a nested guest
we save on the order of 5us per access.
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_64_mmu_hv.c

@@ -441,6 +441,24 @@ int kvmppc_hv_emulate_mmio(struct kvm_run *run, struct kvm_vcpu *vcpu,
 {
 	u32 last_inst;
 
+	/*
+	 * Fast path - check if the guest physical address corresponds to a
+	 * device on the FAST_MMIO_BUS, if so we can avoid loading the
+	 * instruction all together, then we can just handle it and return.
+	 */
+	if (is_store) {
+		int idx, ret;
+
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
+		ret = kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, (gpa_t) gpa, 0,
+				       NULL);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
+		if (!ret) {
+			kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
+			return RESUME_GUEST;
+		}
+	}
+
 	/*
 	 * If we fail, we just return to the guest and try executing it again.
 	 */