linux-kernel - [PATCH 1/2] KVM: x86: leave kvm.ko out of the build if no vendor module is requested

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Message-ID: <20241003230806.229001-2-pbonzini@redhat.com>
Date: Thu,  3 Oct 2024 19:08:05 -0400
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	torvalds@...ux-foundation.org
Subject: [PATCH 1/2] KVM: x86: leave kvm.ko out of the build if no vendor module is requested

kvm.ko is nothing but library code shared by kvm-intel.ko and kvm-amd.ko.
It provides no functionality on its own and it is unnecessary unless one
of the vendor-specific module is compiled.  In particular, /dev/kvm is
not created until one of kvm-intel.ko or kvm-amd.ko is loaded.

Use CONFIG_KVM to decide if it is built-in or a module, but use the
vendor-specific modules for the actual decision on whether to build it.

This also fixes a build failure when CONFIG_KVM_INTEL and CONFIG_KVM_AMD
are both disabled.  The cpu_emergency_register_virt_callback() function
is called from kvm.ko, but it is only defined if at least one of
CONFIG_KVM_INTEL and CONFIG_KVM_AMD is provided.

Fixes: 590b09b1d88e ("KVM: x86: Register "emergency disable" callbacks when virt is enabled")
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/Kconfig  | 7 +++++--
 arch/x86/kvm/Makefile | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 730c2f34d347..81bce9dd9331 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -17,8 +17,8 @@ menuconfig VIRTUALIZATION
 
 if VIRTUALIZATION
 
-config KVM
-	tristate "Kernel-based Virtual Machine (KVM) support"
+config KVM_X86_COMMON
+	def_tristate KVM if KVM_INTEL || KVM_AMD
 	depends on X86_LOCAL_APIC
 	select KVM_COMMON
 	select KVM_GENERIC_MMU_NOTIFIER
@@ -45,6 +45,9 @@ config KVM
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select KVM_GENERIC_PRE_FAULT_MEMORY
 	select KVM_WERROR if WERROR
+
+config KVM
+	tristate "Kernel-based Virtual Machine (KVM) support"
 	help
 	  Support hosting fully virtualized guest machines using hardware
 	  virtualization extensions.  You will need a fairly recent
diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
index 5494669a055a..0d050cfaffb2 100644
--- a/arch/x86/kvm/Makefile
+++ b/arch/x86/kvm/Makefile
@@ -32,7 +32,7 @@ kvm-intel-y		+= vmx/vmx_onhyperv.o vmx/hyperv_evmcs.o
 kvm-amd-y		+= svm/svm_onhyperv.o
 endif
 
-obj-$(CONFIG_KVM)	+= kvm.o
+obj-$(CONFIG_KVM_X86_COMMON)	+= kvm.o
 obj-$(CONFIG_KVM_INTEL)	+= kvm-intel.o
 obj-$(CONFIG_KVM_AMD)	+= kvm-amd.o
 
-- 
2.43.5