[v3] KVM: make uevents configurable

Commit Message

Bernhard Kauer Dec. 5, 2024, 12:27 p.m. UTC

Handling of uevents in userlevel is a bottleneck for tiny VMs.

Running 10_000 VMs keeps one and a half cores busy for 5.4 seconds to let
systemd-udevd handle all messages.  That is roughly 27x longer than
the 0.2 seconds needed for running the VMs without them.

We choose a module parameter here due to its simplicity and ease of
maintenance.

Signed-off-by: Bernhard Kauer <[e-mail]>
---

v1->v2: read-write parameter to avoid killing all running VMs to toggle it
v2->v3: invert the logic, add documentation

 Documentation/admin-guide/kernel-parameters.txt | 4 ++++
 virt/kvm/kvm_main.c                             | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

Patch

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d401577b5a6a..8ae3dc4f2392 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2723,6 +2723,10 @@ 
 			If the value is 0 (the default), KVM will pick a period based
 			on the ratio, such that a page is zapped after 1 hour on average.
 
+	kvm.uevent_notify=
+			[KVM] Send a uevent message to udev whenever a VM is
+			created or destroyed. Default is true.
+
 	kvm-amd.nested=	[KVM,AMD] Control nested virtualization feature in
 			KVM/SVM. Default is 1 (enabled).
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 609e0bd68e8e..276c98e2c37d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -97,6 +97,9 @@  EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
 bool debugfs_per_vm = true;
 module_param(debugfs_per_vm, bool, 0644);
 
+static bool uevent_notify = true;
+module_param(uevent_notify, bool, 0644);
+
 /*
  * Ordering of locks:
  *
@@ -6276,7 +6279,7 @@  static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
 	struct kobj_uevent_env *env;
 	unsigned long long created, active;
 
-	if (!kvm_dev.this_device || !kvm)
+	if (!kvm_dev.this_device || !kvm || !uevent_notify)
 		return;
 
 	mutex_lock(&kvm_lock);