linux-kernel - [PATCH] KVM: x86: allow compiling out the Xen hypercall interface

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Date:   Fri, 26 Feb 2021 05:14:21 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Christoph Hellwig <hch@....de>, David Woodhouse <dwmw@...zon.co.uk>
Subject: [PATCH] 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@....de>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/Kconfig  |  9 ++++++++
 arch/x86/kvm/Makefile |  3 ++-
 arch/x86/kvm/x86.c    |  2 ++
 arch/x86/kvm/xen.h    | 49 ++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 7ac592664c52..5a0d704581bd 100644
--- a/arch/x86/kvm/Kconfig
+++ b/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 && IA32_FEAT_CTL
+	help
+	  Provides KVM support for the Xen shared information page and
+	  for passing Xen hypercalls to userspace.
+
+	  If in doubt, say "N".
+
 config KVM_MMU_AUDIT
 	bool "Audit KVM MMU"
 	depends on KVM && TRACEPOINTS
diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
index aeab168c5711..1b4766fe1de2 100644
--- a/arch/x86/kvm/Makefile
+++ b/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
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bfc928495bd4..9727295b7817 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8039,8 +8039,10 @@ 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)
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index b66a921776f4..6abdbebb029b 100644
--- a/arch/x86/kvm/xen.h
+++ b/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 +38,53 @@ static inline int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu)
 
 	return 0;
 }
+#else
+static inline int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
+{
+	return -EINVAL;
+}
+
+static inline int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
+{
+	return -EINVAL;
+}
+
+static inline int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
+{
+	return -EINVAL;
+}
+
+static inline int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
+{
+	return -EINVAL;
+}
+
+static inline int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
+{
+	return 1;
+}
+
+static inline int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
+{
+	return -EINVAL;
+}
+
+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>
-- 
2.26.2