Re: [PATCH] kvm: use #error instead of BUILD_BUG_ON

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 11, 2026, Alexey Dobriyan wrote:
> Force preprocessing error instead of expanding BUILD_BUG_ON macro and
> forcing compile error at later stage.

This doesn't actually help in practice though, because KVM still uses the missing
macro and so the output is spammed with a bunch of unhelpful errors.

I'm tempted to just delete the guards since they so rarely have meaning, but
vmcs_shadow_fields.h subtly _needs_ the guard since it will #define the missing
macro (unlikes the ops headers, it only requires one to be defined).

And having a guard for vmcs_shadow_fields.h but not the ops headers would be
weird and confusing, so this?

--
From: Sean Christopherson <seanjc@google.com>
Date: Thu, 12 Feb 2026 10:48:34 -0800
Subject: [PATCH] KVM: x86: Immediately fail the build when possible if
 required #define is missing

Guard usage of the must-be-defined macros in KVM's multi-include headers
with the existing #ifdefs that attempt to alert the developer to a missing
macro, and spit out an explicit #error message if a macro is missing, as
referencing the missing macro completely defeats the purpose of the #ifdef
(the compiler spews a ton of error messages and buries the targeted error
message).

Suggested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm-x86-ops.h     | 10 ++++++----
 arch/x86/include/asm/kvm-x86-pmu-ops.h |  8 +++++---
 arch/x86/kvm/vmx/vmcs_shadow_fields.h  |  5 +++--
 3 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index de709fb5bd76..3776cf5382a2 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -1,8 +1,9 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-#if !defined(KVM_X86_OP) || !defined(KVM_X86_OP_OPTIONAL)
-BUILD_BUG_ON(1)
-#endif
-
+#if !defined(KVM_X86_OP) || \
+    !defined(KVM_X86_OP_OPTIONAL) || \
+    !defined(KVM_X86_OP_OPTIONAL_RET0)
+#error Missing one or more KVM_X86_OP #defines
+#else
 /*
  * KVM_X86_OP() and KVM_X86_OP_OPTIONAL() are used to help generate
  * both DECLARE/DEFINE_STATIC_CALL() invocations and
@@ -148,6 +149,7 @@ KVM_X86_OP_OPTIONAL(alloc_apic_backing_page)
 KVM_X86_OP_OPTIONAL_RET0(gmem_prepare)
 KVM_X86_OP_OPTIONAL_RET0(gmem_max_mapping_level)
 KVM_X86_OP_OPTIONAL(gmem_invalidate)
+#endif
 
 #undef KVM_X86_OP
 #undef KVM_X86_OP_OPTIONAL
diff --git a/arch/x86/include/asm/kvm-x86-pmu-ops.h b/arch/x86/include/asm/kvm-x86-pmu-ops.h
index f0aa6996811f..d5452b3433b7 100644
--- a/arch/x86/include/asm/kvm-x86-pmu-ops.h
+++ b/arch/x86/include/asm/kvm-x86-pmu-ops.h
@@ -1,7 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-#if !defined(KVM_X86_PMU_OP) || !defined(KVM_X86_PMU_OP_OPTIONAL)
-BUILD_BUG_ON(1)
-#endif
+#if !defined(KVM_X86_PMU_OP) || \
+    !defined(KVM_X86_PMU_OP_OPTIONAL)
+#error Missing one or more KVM_X86_PMU_OP #defines
+#else
 
 /*
  * KVM_X86_PMU_OP() and KVM_X86_PMU_OP_OPTIONAL() are used to help generate
@@ -26,6 +27,7 @@ KVM_X86_PMU_OP_OPTIONAL(cleanup)
 KVM_X86_PMU_OP_OPTIONAL(write_global_ctrl)
 KVM_X86_PMU_OP(mediated_load)
 KVM_X86_PMU_OP(mediated_put)
+#endif
 
 #undef KVM_X86_PMU_OP
 #undef KVM_X86_PMU_OP_OPTIONAL
diff --git a/arch/x86/kvm/vmx/vmcs_shadow_fields.h b/arch/x86/kvm/vmx/vmcs_shadow_fields.h
index cad128d1657b..67e821c2be6d 100644
--- a/arch/x86/kvm/vmx/vmcs_shadow_fields.h
+++ b/arch/x86/kvm/vmx/vmcs_shadow_fields.h
@@ -1,6 +1,6 @@
 #if !defined(SHADOW_FIELD_RO) && !defined(SHADOW_FIELD_RW)
-BUILD_BUG_ON(1)
-#endif
+#error Must #define at least one of SHADOW_FIELD_RO or SHADOW_FIELD_RW
+#else
 
 #ifndef SHADOW_FIELD_RO
 #define SHADOW_FIELD_RO(x, y)
@@ -74,6 +74,7 @@ SHADOW_FIELD_RW(HOST_GS_BASE, host_gs_base)
 /* 64-bit */
 SHADOW_FIELD_RO(GUEST_PHYSICAL_ADDRESS, guest_physical_address)
 SHADOW_FIELD_RO(GUEST_PHYSICAL_ADDRESS_HIGH, guest_physical_address)
+#endif
 
 #undef SHADOW_FIELD_RO
 #undef SHADOW_FIELD_RW

base-commit: 183bb0ce8c77b0fd1fb25874112bc8751a461e49
--