[PATCH v2 3/6] KVM: Add a module param to allow enabling virtualization when KVM is loaded

[Sean Christopherson <seanjc@xxxxxxxxxx>]

Add an off-by-default module param, enable_virt_at_load, to let userspace
force virtualization to be enabled in hardware when KVM is initialized,
i.e. just before /dev/kvm is exposed to userspace.  Enabling virtualization
during KVM initialization allows userspace to avoid the additional latency
when creating/destroying the first/last VM.  Now that KVM uses the cpuhp
framework to do per-CPU enabling, the latency could be non-trivial as the
cpuhup bringup/teardown is serialized across CPUs, e.g. the latency could
be problematic for use case that need to spin up VMs quickly.

Enabling virtualizaton during initialization will also allow KVM to setup
the Intel TDX Module, which requires VMX to be fully enabled, without
needing additional APIs to temporarily enable virtualization.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8ba2861e7788..8d331ab3aef7 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5495,6 +5495,9 @@ static struct miscdevice kvm_dev = {
 };
 
 #ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
+static bool enable_virt_at_load;
+module_param(enable_virt_at_load, bool, 0444);
+
 __visible bool kvm_rebooting;
 EXPORT_SYMBOL_GPL(kvm_rebooting);
 
@@ -5645,15 +5648,41 @@ static void kvm_disable_virtualization(void)
 	unregister_syscore_ops(&kvm_syscore_ops);
 	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
 }
+
+static int kvm_init_virtualization(void)
+{
+	if (enable_virt_at_load)
+		return kvm_enable_virtualization();
+
+	return 0;
+}
+
+static void kvm_uninit_virtualization(void)
+{
+	if (enable_virt_at_load)
+		kvm_disable_virtualization();
+
+	WARN_ON(kvm_usage_count);
+}
 #else /* CONFIG_KVM_GENERIC_HARDWARE_ENABLING */
 static int kvm_enable_virtualization(void)
 {
 	return 0;
 }
 
+static int kvm_init_virtualization(void)
+{
+	return 0;
+}
+
 static void kvm_disable_virtualization(void)
 {
 
+}
+
+static void kvm_uninit_virtualization(void)
+{
+
 }
 #endif /* CONFIG_KVM_GENERIC_HARDWARE_ENABLING */
 
@@ -6395,6 +6424,10 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 
 	kvm_gmem_init(module);
 
+	r = kvm_init_virtualization();
+	if (r)
+		goto err_virt;
+
 	/*
 	 * Registration _must_ be the very last thing done, as this exposes
 	 * /dev/kvm to userspace, i.e. all infrastructure must be setup!
@@ -6408,6 +6441,8 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 	return 0;
 
 err_register:
+	kvm_uninit_virtualization();
+err_virt:
 	kvm_vfio_ops_exit();
 err_vfio:
 	kvm_async_pf_deinit();
@@ -6433,6 +6468,8 @@ void kvm_exit(void)
 	 */
 	misc_deregister(&kvm_dev);
 
+	kvm_uninit_virtualization();
+
 	debugfs_remove_recursive(kvm_debugfs_dir);
 	for_each_possible_cpu(cpu)
 		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
-- 
2.45.0.215.g3402c0e53f-goog