Re: [PATCH] KVM: x86: Immediately fail the build when possible if required #define is missing

[Yao Yuan]

On Mon, Mar 02, 2026 at 01:26:19PM +0800, Sean Christopherson wrote:
> 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@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  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

More clear than the BUILD_BUG_ON(1) to me.

Reviewed-by: Yuan Yao <yaoyuan@xxxxxxxxxxxxxxxxx>

> +#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
> --
> 2.53.0.473.g4a7958ca14-goog
>