KVM: x86/mmu: Replace MMU_DEBUG with proper KVM_PROVE_MMU Kconfig

Replace MMU_DEBUG, which requires manually modifying KVM to enable the
macro, with a proper Kconfig, KVM_PROVE_MMU.  Now that pgprintk() and
rmap_printk() are gone, i.e. the macro guards only KVM_MMU_WARN_ON() and
won't flood the kernel logs, enabling the option for debug kernels is both
desirable and feasible.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230729004722.1056172-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/Kconfig

@@ -138,6 +138,19 @@ config KVM_XEN
 
 	  If in doubt, say "N".
 
+config KVM_PROVE_MMU
+	bool "Prove KVM MMU correctness"
+	depends on DEBUG_KERNEL
+	depends on KVM
+	depends on EXPERT
+	help
+	  Enables runtime assertions in KVM's MMU that are too costly to enable
+	  in anything remotely resembling a production environment, e.g. this
+	  gates code that verifies a to-be-freed page table doesn't have any
+	  present SPTEs.
+
+	  If in doubt, say "N".
+
 config KVM_EXTERNAL_WRITE_TRACKING
 	bool
 

arch/x86/kvm/mmu/mmu.c

@@ -1691,7 +1691,7 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 
 static void kvm_mmu_check_sptes_at_free(struct kvm_mmu_page *sp)
 {
-#ifdef MMU_DEBUG
+#ifdef CONFIG_KVM_PROVE_MMU
 	int i;
 
 	for (i = 0; i < SPTE_ENT_PER_PAGE; i++) {

arch/x86/kvm/mmu/mmu_internal.h

@@ -6,9 +6,7 @@
 #include <linux/kvm_host.h>
 #include <asm/kvm_host.h>
 
-#undef MMU_DEBUG
-
-#ifdef MMU_DEBUG
+#ifdef CONFIG_KVM_PROVE_MMU
 #define KVM_MMU_WARN_ON(x) WARN_ON_ONCE(x)
 #else
 #define KVM_MMU_WARN_ON(x) do { } while (0)