[v4 0/3] KVM: irqchip: synchronize srcu only if needed

[v4 0/3] KVM: irqchip: synchronize srcu only if needed

[Yi Wang]

From: Yi Wang <foxywang@tencent.com>

We found that it may cost more than 20 milliseconds very accidentally
to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
already.

The reason is that when vmm(qemu/CloudHypervisor) invokes
KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
might_sleep and kworker of srcu may cost some delay during this period.
One way makes sence is setup empty irq routing when creating vm and
so that x86/s390 don't need to setup empty/dummy irq routing.

Note: I have no s390 machine so this patch has not been tested
thoroughly on s390 platform. Thanks to Christian for a quick test on
s390 and it still seems to work[1].

Changelog:
----------
v4:
  - replace loop with memset when setup empty irq routing table.

v3:
  - squash setup empty routing function and use of that into one commit
  - drop the comment in s390 part

v2:
  - setup empty irq routing in kvm_create_vm
  - don't setup irq routing in x86 KVM_CAP_SPLIT_IRQCHIP
  - don't setup irq routing in s390 KVM_CREATE_IRQCHIP

v1:
  https://lore.kernel.org/kvm/20240112091128.3868059-1-foxywang@tencent.com/

1. https://lore.kernel.org/lkml/f898e36f-ba02-4c52-a3be-06caac13323e@linux.ibm.com/


Yi Wang (3):
  KVM: setup empty irq routing when create vm
  KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP
  KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP

 arch/s390/kvm/kvm-s390.c |  9 +--------
 arch/x86/kvm/irq.h       |  1 -
 arch/x86/kvm/irq_comm.c  |  5 -----
 arch/x86/kvm/x86.c       |  3 ---
 include/linux/kvm_host.h |  1 +
 virt/kvm/irqchip.c       | 19 +++++++++++++++++++
 virt/kvm/kvm_main.c      |  4 ++++
 7 files changed, 25 insertions(+), 17 deletions(-)

-- 
2.39.3

[v4 1/3] KVM: setup empty irq routing when create vm

[Yi Wang]

From: Yi Wang <foxywang@tencent.com>

Add a new function to setup empty irq routing in kvm path, which
can be invoded in non-architecture-specific functions. The difference
compared to the kvm_setup_empty_irq_routing() is this function just
alloc the empty irq routing and does not need synchronize srcu, as
we will call it in kvm_create_vm().

Using the new adding function, we can setup empty irq routing when
kvm_create_vm(), so that x86 and s390 no longer need to set
empty/dummy irq routing when creating an IRQCHIP 'cause it avoid
an synchronize_srcu.

Signed-off-by: Yi Wang <foxywang@tencent.com>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/irqchip.c       | 19 +++++++++++++++++++
 virt/kvm/kvm_main.c      |  4 ++++
 3 files changed, 24 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 179df96b20f8..48b5d7fc108d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2068,6 +2068,7 @@ int kvm_set_irq_routing(struct kvm *kvm,
 			const struct kvm_irq_routing_entry *entries,
 			unsigned nr,
 			unsigned flags);
+int kvm_setup_empty_irq_routing_lockless(struct kvm *kvm);
 int kvm_set_routing_entry(struct kvm *kvm,
 			  struct kvm_kernel_irq_routing_entry *e,
 			  const struct kvm_irq_routing_entry *ue);
diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
index 1e567d1f6d3d..266bab99a8a8 100644
--- a/virt/kvm/irqchip.c
+++ b/virt/kvm/irqchip.c
@@ -237,3 +237,22 @@ int kvm_set_irq_routing(struct kvm *kvm,
 
 	return r;
 }
+
+int kvm_setup_empty_irq_routing_lockless(struct kvm *kvm)
+{
+	struct kvm_irq_routing_table *new;
+	int chip_size;
+
+	new = kzalloc(struct_size(new, map, 1), GFP_KERNEL_ACCOUNT);
+	if (!new)
+		return -ENOMEM;
+
+	new->nr_rt_entries = 1;
+
+	chip_size = sizeof(int) * KVM_NR_IRQCHIPS * KVM_IRQCHIP_NUM_PINS;
+	memset(new->chip, -1, chip_size);
+
+	RCU_INIT_POINTER(kvm->irq_routing, new);
+
+	return 0;
+}
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0f50960b0e3a..3438d6aa0f23 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1288,6 +1288,10 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 	if (r)
 		goto out_err;
 
+	r = kvm_setup_empty_irq_routing_lockless(kvm);
+	if (r)
+		goto out_err;
+
 	mutex_lock(&kvm_lock);
 	list_add(&kvm->vm_list, &vm_list);
 	mutex_unlock(&kvm_lock);
-- 
2.39.3

[v4 2/3] KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP

[Yi Wang]

From: Yi Wang <foxywang@tencent.com>

We found that it may cost more than 20 milliseconds very accidentally
to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
already.

The reason is that when vmm(qemu/CloudHypervisor) invokes
KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
might_sleep and kworker of srcu may cost some delay during this period.

As we have set up empty irq routing when creating vm, so this is no
need now.

Signed-off-by: Yi Wang <foxywang@tencent.com>
---
 arch/x86/kvm/irq.h      | 1 -
 arch/x86/kvm/irq_comm.c | 5 -----
 arch/x86/kvm/x86.c      | 3 ---
 3 files changed, 9 deletions(-)

diff --git a/arch/x86/kvm/irq.h b/arch/x86/kvm/irq.h
index c2d7cfe82d00..76d46b2f41dd 100644
--- a/arch/x86/kvm/irq.h
+++ b/arch/x86/kvm/irq.h
@@ -106,7 +106,6 @@ void __kvm_migrate_timers(struct kvm_vcpu *vcpu);
 int apic_has_pending_timer(struct kvm_vcpu *vcpu);
 
 int kvm_setup_default_irq_routing(struct kvm *kvm);
-int kvm_setup_empty_irq_routing(struct kvm *kvm);
 int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
 			     struct kvm_lapic_irq *irq,
 			     struct dest_map *dest_map);
diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
index 68f3f6c26046..6ee7ca39466e 100644
--- a/arch/x86/kvm/irq_comm.c
+++ b/arch/x86/kvm/irq_comm.c
@@ -397,11 +397,6 @@ int kvm_setup_default_irq_routing(struct kvm *kvm)
 
 static const struct kvm_irq_routing_entry empty_routing[] = {};
 
-int kvm_setup_empty_irq_routing(struct kvm *kvm)
-{
-	return kvm_set_irq_routing(kvm, empty_routing, 0, 0);
-}
-
 void kvm_arch_post_irq_routing_update(struct kvm *kvm)
 {
 	if (!irqchip_split(kvm))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e02cc710f56d..eee3a0892137 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6469,9 +6469,6 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			goto split_irqchip_unlock;
 		if (kvm->created_vcpus)
 			goto split_irqchip_unlock;
-		r = kvm_setup_empty_irq_routing(kvm);
-		if (r)
-			goto split_irqchip_unlock;
 		/* Pairs with irqchip_in_kernel. */
 		smp_wmb();
 		kvm->arch.irqchip_mode = KVM_IRQCHIP_SPLIT;
-- 
2.39.3

[v4 3/3] KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP

[Yi Wang]

From: Yi Wang <foxywang@tencent.com>

As we have setup empty irq routing in kvm_create_vm(), there's
no need to setup dummy routing when KVM_CREATE_IRQCHIP.

Signed-off-by: Yi Wang <foxywang@tencent.com>
---
 arch/s390/kvm/kvm-s390.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index ea63ac769889..78a1e2d801ba 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2998,14 +2998,7 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		break;
 	}
 	case KVM_CREATE_IRQCHIP: {
-		struct kvm_irq_routing_entry routing;
-
-		r = -EINVAL;
-		if (kvm->arch.use_irqchip) {
-			/* Set up dummy routing. */
-			memset(&routing, 0, sizeof(routing));
-			r = kvm_set_irq_routing(kvm, &routing, 0, 0);
-		}
+		r = 0;
 		break;
 	}
 	case KVM_SET_DEVICE_ATTR: {
-- 
2.39.3