[PATCH 2/2] KVM: x86: Sanity check that kvm_has_noapic_vcpu is zero at module_exit()

From: Sean Christopherson
Date: Fri Feb 09 2024 - 17:23:09 EST

WARN if kvm.ko is unloaded with an elevated kvm_has_noapic_vcpu to guard
against incorrect management of the key, e.g. to detect if KVM fails to
decrement the key in error paths. Because kvm_has_noapic_vcpu is purely
an optimization, in all likelihood KVM could completely botch handling of
kvm_has_noapic_vcpu and no one would notice (which is a good argument for
deleting the key entirely, but that's a problem for another day).

Note, ideally the sanity check would be performance when kvm_usage_count
goes to zero, but adding an arch callback just for this sanity check isn't
at all worth doing.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
arch/x86/kvm/x86.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 59119157bd20..779a5b32a5bf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13878,9 +13878,6 @@ module_init(kvm_x86_init);

static void __exit kvm_x86_exit(void)
- /*
- * If module_init() is implemented, module_exit() must also be
- * implemented to allow module unload.
- */
+ WARN_ON_ONCE(static_branch_unlikely(&kvm_has_noapic_vcpu));