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: 590b09b1 ("KVM: x86: Register "emergency disable" callbacks when virt is enabled")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/Kconfig

@@ -17,8 +17,8 @@ menuconfig VIRTUALIZATION
 
 if VIRTUALIZATION
 
-config KVM
-	tristate "Kernel-based Virtual Machine (KVM) support"
+config KVM_X86
+	def_tristate KVM if KVM_INTEL || KVM_AMD
 	depends on X86_LOCAL_APIC
 	select KVM_COMMON
 	select KVM_GENERIC_MMU_NOTIFIER
@@ -44,7 +44,11 @@ config KVM
 	select HAVE_KVM_PM_NOTIFIER if PM
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select KVM_GENERIC_PRE_FAULT_MEMORY
+	select KVM_GENERIC_PRIVATE_MEM if KVM_SW_PROTECTED_VM
 	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
@@ -77,7 +81,6 @@ config KVM_SW_PROTECTED_VM
 	bool "Enable support for KVM software-protected VMs"
 	depends on EXPERT
 	depends on KVM && X86_64
-	select KVM_GENERIC_PRIVATE_MEM
 	help
 	  Enable support for KVM software-protected VMs.  Currently, software-
 	  protected VMs are purely a development and testing vehicle for

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)	+= kvm.o
 obj-$(CONFIG_KVM_INTEL)	+= kvm-intel.o
 obj-$(CONFIG_KVM_AMD)	+= kvm-amd.o