KVM: x86: allow compiling out the Xen hypercall interface

The Xen hypercall interface adds to the attack surface of the hypervisor
and will be used quite rarely.  Allow compiling it out.

Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/Kconfig

@@ -103,6 +103,15 @@ config KVM_AMD_SEV
 	  Provides support for launching Encrypted VMs (SEV) and Encrypted VMs
 	  with Encrypted State (SEV-ES) on AMD processors.
 
+config KVM_XEN
+	bool "Support for Xen hypercall interface"
+	depends on KVM
+	help
+	  Provides KVM support for the hosting Xen HVM guests and
+	  passing Xen hypercalls to userspace.
+
+	  If in doubt, say "N".
+
 config KVM_MMU_AUDIT
 	bool "Audit KVM MMU"
 	depends on KVM && TRACEPOINTS

arch/x86/kvm/Makefile

@@ -14,11 +14,12 @@ kvm-y			+= $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o \
 				$(KVM)/dirty_ring.o
 kvm-$(CONFIG_KVM_ASYNC_PF)	+= $(KVM)/async_pf.o
 
-kvm-y			+= x86.o emulate.o i8259.o irq.o lapic.o xen.o \
+kvm-y			+= x86.o emulate.o i8259.o irq.o lapic.o \
 			   i8254.o ioapic.o irq_comm.o cpuid.o pmu.o mtrr.o \
 			   hyperv.o debugfs.o mmu/mmu.o mmu/page_track.o \
 			   mmu/spte.o
 kvm-$(CONFIG_X86_64) += mmu/tdp_iter.o mmu/tdp_mmu.o
+kvm-$(CONFIG_KVM_XEN)	+= xen.o
 
 kvm-intel-y		+= vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
 			   vmx/evmcs.o vmx/nested.o vmx/posted_intr.o

arch/x86/kvm/x86.c

@@ -3755,11 +3755,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
 		r = 1;
 		break;
+#ifdef CONFIG_KVM_XEN
 	case KVM_CAP_XEN_HVM:
 		r = KVM_XEN_HVM_CONFIG_HYPERCALL_MSR |
 		    KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL |
 		    KVM_XEN_HVM_CONFIG_SHARED_INFO;
 		break;
+#endif
 	case KVM_CAP_SYNC_REGS:
 		r = KVM_SYNC_X86_VALID_FIELDS;
 		break;
@@ -5012,6 +5014,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	case KVM_GET_SUPPORTED_HV_CPUID:
 		r = kvm_ioctl_get_supported_hv_cpuid(vcpu, argp);
 		break;
+#ifdef CONFIG_KVM_XEN
 	case KVM_XEN_VCPU_GET_ATTR: {
 		struct kvm_xen_vcpu_attr xva;
 
@@ -5032,6 +5035,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		r = kvm_xen_vcpu_set_attr(vcpu, &xva);
 		break;
 	}
+#endif
 	default:
 		r = -EINVAL;
 	}
@@ -5653,6 +5657,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
 			kvm->arch.bsp_vcpu_id = arg;
 		mutex_unlock(&kvm->lock);
 		break;
+#ifdef CONFIG_KVM_XEN
 	case KVM_XEN_HVM_CONFIG: {
 		struct kvm_xen_hvm_config xhc;
 		r = -EFAULT;
@@ -5681,6 +5686,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
 		r = kvm_xen_hvm_set_attr(kvm, &xha);
 		break;
 	}
+#endif
 	case KVM_SET_CLOCK: {
 		struct kvm_clock_data user_ns;
 		u64 now_ns;
@@ -8039,8 +8045,10 @@ void kvm_arch_exit(void)
 	kvm_mmu_module_exit();
 	free_percpu(user_return_msrs);
 	kmem_cache_destroy(x86_fpu_cache);
+#ifdef CONFIG_KVM_XEN
 	static_key_deferred_flush(&kvm_xen_enabled);
 	WARN_ON(static_branch_unlikely(&kvm_xen_enabled.key));
+#endif
 }
 
 static int __kvm_vcpu_halt(struct kvm_vcpu *vcpu, int state, int reason)

arch/x86/kvm/xen.h

@@ -9,6 +9,7 @@
 #ifndef __ARCH_X86_KVM_XEN_H__
 #define __ARCH_X86_KVM_XEN_H__
 
+#ifdef CONFIG_KVM_XEN
 #include <linux/jump_label_ratelimit.h>
 
 extern struct static_key_false_deferred kvm_xen_enabled;
@@ -18,7 +19,6 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data);
 int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
 int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
-int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
 int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data);
 int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc);
 void kvm_xen_destroy_vm(struct kvm *kvm);
@@ -38,6 +38,28 @@ static inline int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu)
 
 	return 0;
 }
+#else
+static inline int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
+{
+	return 1;
+}
+
+static inline void kvm_xen_destroy_vm(struct kvm *kvm)
+{
+}
+
+static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
+{
+	return false;
+}
+
+static inline int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+#endif
+
+int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
 
 /* 32-bit compatibility definitions, also used natively in 32-bit build */
 #include <asm/pvclock-abi.h>