[PATCH] kvm/irqchip: Speed up KVM_SET_GSI_ROUTING

[Christian Borntraeger <borntraeger@de.ibm.com>]

When starting lots of dataplane devices the bootup takes very long on my
s390 system(prototype irqfd code). With larger setups we are even able to
trigger some timeouts in some components.
Turns out that the KVM_SET_GSI_ROUTING ioctl takes very
long (strace claims up to 0.1 sec) when having multiple CPUs.
This is caused by the  synchronize_rcu and the HZ=100 of s390.
We can defer the freeing outside of the ioctl path by using kfree_rcu.

Please note that we now have to check for a NULL pointer, since
the underlying call_rcu in kfree_rcu cannot handle broken rcu
callbacks.

This patch reduces the boot time till mounting root from 8 to 2 seconds
on my s390 guest with 100 disks.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 include/linux/kvm_host.h | 1 +
 virt/kvm/irqchip.c       | 5 ++---
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3d1b0e6..2f19079 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -327,6 +327,7 @@ struct kvm_kernel_irq_routing_entry {
 #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
 
 struct kvm_irq_routing_table {
+	struct rcu_head	rcu;
 	int chip[KVM_NR_IRQCHIPS][KVM_IRQCHIP_NUM_PINS];
 	struct kvm_kernel_irq_routing_entry *rt_entries;
 	u32 nr_rt_entries;
diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
index 20dc9e4..3e2ebed 100644
--- a/virt/kvm/irqchip.c
+++ b/virt/kvm/irqchip.c
@@ -226,12 +226,11 @@ int kvm_set_irq_routing(struct kvm *kvm,
 	kvm_irq_routing_update(kvm, new);
 	mutex_unlock(&kvm->irq_lock);
 
-	synchronize_rcu();
-
 	new = old;
 	r = 0;
 
 out:
-	kfree(new);
+	if (new)
+		kfree_rcu(new, rcu);
 	return r;
 }
-- 
1.8.4.2