Re: [RFC PATCH v5 04/45] KVM: x86: Make "external SPTE" ops that can fail RET0 static calls

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 29, 2026, Rick P Edgecombe wrote:
> On Wed, 2026-01-28 at 17:14 -0800, Sean Christopherson wrote:
> > Define kvm_x86_ops .link_external_spt(), .set_external_spte(), and
> > .free_external_spt() as RET0 static calls so that an unexpected call to a
> > a default operation doesn't consume garbage.
> > 
> > Fixes: 77ac7079e66d ("KVM: x86/tdp_mmu: Propagate building mirror page tables")
> > Fixes: 94faba8999b9 ("KVM: x86/tdp_mmu: Propagate tearing down mirror page tables")
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> 
> We don't want to crash unnecessarily, but do we want to get some sort of notice?

Hmm, that's probably doable, but definitely in a separate patch.  E.g. something
like:

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 6083fb07cd3b..270149f84bb4 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -3,6 +3,13 @@
 BUILD_BUG_ON(1)
 #endif
 
+#ifndef KVM_X86_OP_OPTIONAL
+#define KVM_X86_OP_OPTIONAL KVM_X86_OP
+#define KVM_X86_OP_OPTIONAL_RET0 KVM_X86_OP
+#define KVM_X86_OP_OPTIONAL_WARN KVM_X86_OP
+#define KVM_X86_OP_OPTIONAL_RET0_WARN KVM_X86_OP
+#endif
+
 /*
  * KVM_X86_OP() and KVM_X86_OP_OPTIONAL() are used to help generate
  * both DECLARE/DEFINE_STATIC_CALL() invocations and
@@ -94,11 +101,11 @@ KVM_X86_OP_OPTIONAL_RET0(set_tss_addr)
 KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
 KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
 KVM_X86_OP(load_mmu_pgd)
-KVM_X86_OP_OPTIONAL(alloc_external_sp)
-KVM_X86_OP_OPTIONAL(free_external_sp)
-KVM_X86_OP_OPTIONAL_RET0(set_external_spte)
-KVM_X86_OP_OPTIONAL(reclaim_external_sp)
-KVM_X86_OP_OPTIONAL_RET0(topup_external_cache)
+KVM_X86_OP_OPTIONAL_WARN(alloc_external_sp)
+KVM_X86_OP_OPTIONAL_WARN(free_external_sp)
+KVM_X86_OP_OPTIONAL_RET0_WARN(set_external_spte)
+KVM_X86_OP_OPTIONAL_WARN(reclaim_external_sp)
+KVM_X86_OP_OPTIONAL_RET0_WARN(topup_external_cache)
 KVM_X86_OP(has_wbinvd_exit)
 KVM_X86_OP(get_l2_tsc_offset)
 KVM_X86_OP(get_l2_tsc_multiplier)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index cd3e7dc6ab9b..663c9943c0dd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2004,8 +2004,6 @@ extern struct kvm_x86_ops kvm_x86_ops;
 
 #define KVM_X86_OP(func) \
        DECLARE_STATIC_CALL(kvm_x86_##func, *(((struct kvm_x86_ops *)0)->func));
-#define KVM_X86_OP_OPTIONAL KVM_X86_OP
-#define KVM_X86_OP_OPTIONAL_RET0 KVM_X86_OP
 #include <asm/kvm-x86-ops.h>
 
 int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c3d71ba9a1dc..1748f44c81c0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -143,8 +143,6 @@ struct kvm_x86_ops kvm_x86_ops __read_mostly;
 #define KVM_X86_OP(func)                                            \
        DEFINE_STATIC_CALL_NULL(kvm_x86_##func,                      \
                                *(((struct kvm_x86_ops *)0)->func));
-#define KVM_X86_OP_OPTIONAL KVM_X86_OP
-#define KVM_X86_OP_OPTIONAL_RET0 KVM_X86_OP
 #include <asm/kvm-x86-ops.h>
 EXPORT_STATIC_CALL_GPL(kvm_x86_get_cs_db_l_bits);
 EXPORT_STATIC_CALL_GPL(kvm_x86_cache_reg);
@@ -9965,6 +9963,17 @@ static struct notifier_block pvclock_gtod_notifier = {
 };
 #endif
 
+static void kvm_static_call_warn(void)
+{
+       WARN_ON_ONCE(1);
+}
+
+static long kvm_static_call_warn_return0(void)
+{
+       WARN_ON_ONCE(1);
+       return 0;
+}
+
 static inline void kvm_ops_update(struct kvm_x86_init_ops *ops)
 {
        memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
@@ -9977,6 +9986,12 @@ static inline void kvm_ops_update(struct kvm_x86_init_ops *ops)
 #define KVM_X86_OP_OPTIONAL_RET0(func) \
        static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
                                           (void *)__static_call_return0);
+#define KVM_X86_OP_OPTIONAL_WARN(func) \
+       static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
+                                          (void *)kvm_static_call_warn);
+#define KVM_X86_OP_OPTIONAL_RET0_WARN(func) \
+       static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
+                                          (void *)kvm_static_call_warn_return0);
 #include <asm/kvm-x86-ops.h>
 #undef __KVM_X86_OP