KVM: x86/xen: Add kvm_xen_enabled static key

The code paths for Xen support are all fairly lightweight but if we hide
them behind this, they're even *more* lightweight for any system which
isn't actually hosting Xen guests.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

arch/x86/kvm/x86.c

@@ -7988,6 +7988,7 @@ void kvm_arch_exit(void)
 	kvm_mmu_module_exit();
 	free_percpu(user_return_msrs);
 	kmem_cache_destroy(x86_fpu_cache);
+	WARN_ON(static_branch_unlikely(&kvm_xen_enabled.key));
 }
 
 static int __kvm_vcpu_halt(struct kvm_vcpu *vcpu, int state, int reason)
@@ -10581,6 +10582,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
 	kvm_mmu_uninit_vm(kvm);
 	kvm_page_track_cleanup(kvm);
+	kvm_xen_destroy_vm(kvm);
 	kvm_hv_destroy_vm(kvm);
 }
 

arch/x86/kvm/xen.c

@@ -16,6 +16,8 @@
 
 #include "trace.h"
 
+DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_enabled, HZ);
+
 int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 {
 	struct kvm *kvm = vcpu->kvm;
@@ -93,10 +95,25 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
 	     xhc->blob_size_32 || xhc->blob_size_64))
 		return -EINVAL;
 
+	mutex_lock(&kvm->lock);
+
+	if (xhc->msr && !kvm->arch.xen_hvm_config.msr)
+		static_branch_inc(&kvm_xen_enabled.key);
+	else if (!xhc->msr && kvm->arch.xen_hvm_config.msr)
+		static_branch_slow_dec_deferred(&kvm_xen_enabled);
+
 	memcpy(&kvm->arch.xen_hvm_config, xhc, sizeof(*xhc));
+
+	mutex_unlock(&kvm->lock);
 	return 0;
 }
 
+void kvm_xen_destroy_vm(struct kvm *kvm)
+{
+	if (kvm->arch.xen_hvm_config.msr)
+		static_branch_slow_dec_deferred(&kvm_xen_enabled);
+}
+
 static int kvm_xen_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)
 {
 	kvm_rax_write(vcpu, result);

arch/x86/kvm/xen.h

@@ -9,14 +9,20 @@
 #ifndef __ARCH_X86_KVM_XEN_H__
 #define __ARCH_X86_KVM_XEN_H__
 
+#include <linux/jump_label_ratelimit.h>
+
+extern struct static_key_false_deferred kvm_xen_enabled;
+
 int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
 int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data);
 int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc);
+void kvm_xen_destroy_vm(struct kvm *kvm);
 
 static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
 {
-	return kvm->arch.xen_hvm_config.flags &
-		KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL;
+	return static_branch_unlikely(&kvm_xen_enabled.key) &&
+		(kvm->arch.xen_hvm_config.flags &
+		 KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL);
 }
 
 #endif /* __ARCH_X86_KVM_XEN_H__ */