KVM: PPC: Book3S HV: Add tunable to control H_IPI redirection

Redirecting the wakeup of a VCPU from the H_IPI hypercall to
a core running in the host is usually a good idea, most workloads
seemed to benefit. However, in one heavily interrupt-driven SMT1
workload, some regression was observed. This patch adds a kvm_hv
module parameter called h_ipi_redirect to control this feature.

The default value for this tunable is 1 - that is enable the feature.

Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/include/asm/kvm_ppc.h

@@ -464,6 +464,7 @@ extern int kvmppc_xics_set_icp(struct kvm_vcpu *vcpu, u64 icpval);
 extern int kvmppc_xics_connect_vcpu(struct kvm_device *dev,
 			struct kvm_vcpu *vcpu, u32 cpu);
 extern void kvmppc_xics_ipi_action(void);
+extern int h_ipi_redirect;
 #else
 static inline void kvmppc_alloc_host_rm_ops(void) {};
 static inline void kvmppc_free_host_rm_ops(void) {};

arch/powerpc/kvm/book3s_hv.c

@@ -81,6 +81,17 @@ static int target_smt_mode;
 module_param(target_smt_mode, int, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(target_smt_mode, "Target threads per core (0 = max)");
 
+#ifdef CONFIG_KVM_XICS
+static struct kernel_param_ops module_param_ops = {
+	.set = param_set_int,
+	.get = param_get_int,
+};
+
+module_param_cb(h_ipi_redirect, &module_param_ops, &h_ipi_redirect,
+							S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(h_ipi_redirect, "Redirect H_IPI wakeup to a free host core");
+#endif
+
 static void kvmppc_end_cede(struct kvm_vcpu *vcpu);
 static int kvmppc_hv_setup_htab_rma(struct kvm_vcpu *vcpu);
 

arch/powerpc/kvm/book3s_hv_rm_xics.c

@@ -24,6 +24,9 @@
 
 #define DEBUG_PASSUP
 
+int h_ipi_redirect = 1;
+EXPORT_SYMBOL(h_ipi_redirect);
+
 static void icp_rm_deliver_irq(struct kvmppc_xics *xics, struct kvmppc_icp *icp,
 			    u32 new_irq);
 
@@ -148,7 +151,7 @@ static void icp_rm_set_vcpu_irq(struct kvm_vcpu *vcpu,
 	cpu = vcpu->arch.thread_cpu;
 	if (cpu < 0 || cpu >= nr_cpu_ids) {
 		hcore = -1;
-		if (kvmppc_host_rm_ops_hv)
+		if (kvmppc_host_rm_ops_hv && h_ipi_redirect)
 			hcore = find_available_hostcore(XICS_RM_KICK_VCPU);
 		if (hcore != -1) {
 			icp_send_hcore_msg(hcore, vcpu);