KVM: x86/xen: Add an #ifdef'd helper to detect writes to Xen MSR

Add a helper to detect writes to the Xen hypercall page MSR, and provide a
stub for CONFIG_KVM_XEN=n to optimize out the check for kernels built
without Xen support.

Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20250215011437.1203084-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -3738,7 +3738,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	 * page setup; it could incur locking paths which are not expected
 	 * if userspace sets the MSR in an unusual location.
 	 */
-	if (msr && msr == vcpu->kvm->arch.xen_hvm_config.msr &&
+	if (kvm_xen_is_hypercall_page_msr(vcpu->kvm, msr) &&
 	    !msr_info->host_initiated)
 		return kvm_xen_write_hypercall_page(vcpu, data);
 

arch/x86/kvm/xen.h

@@ -56,6 +56,11 @@ static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
 		kvm->arch.xen_hvm_config.msr;
 }
 
+static inline bool kvm_xen_is_hypercall_page_msr(struct kvm *kvm, u32 msr)
+{
+	return msr && msr == kvm->arch.xen_hvm_config.msr;
+}
+
 static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
 {
 	return static_branch_unlikely(&kvm_xen_enabled.key) &&
@@ -124,6 +129,11 @@ static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
 	return false;
 }
 
+static inline bool kvm_xen_is_hypercall_page_msr(struct kvm *kvm, u32 msr)
+{
+	return false;
+}
+
 static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
 {
 	return false;