diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
index 09edbd23b3..e1c268789e 100644
--- a/xen/arch/x86/hvm/vmx/vmcs.c
+++ b/xen/arch/x86/hvm/vmx/vmcs.c
@@ -781,7 +781,7 @@ static int _vmx_cpu_up(bool bsp)
     return 0;
 }
 
-int cf_check vmx_cpu_up()
+int cf_check vmx_cpu_up(void)
 {
     return _vmx_cpu_up(false);
 }
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 782b11c8a9..4e44a43cc5 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1361,7 +1361,7 @@ s_time_t get_s_time_fixed(u64 at_tsc)
     return t->stamp.local_stime + scale_delta(delta, &t->tsc_scale);
 }
 
-s_time_t get_s_time()
+s_time_t get_s_time(void)
 {
     return get_s_time_fixed(0);
 }
diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
index 921b71e819..0e187f6ae3 100644
--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -606,7 +606,7 @@ int __init iommu_setup(void)
     return rc;
 }
 
-int iommu_suspend()
+int iommu_suspend(void)
 {
     if ( iommu_enabled )
         return iommu_call(iommu_get_ops(), suspend);
@@ -614,7 +614,7 @@ int iommu_suspend()
     return 0;
 }
 
-void iommu_resume()
+void iommu_resume(void)
 {
     if ( iommu_enabled )
         iommu_vcall(iommu_get_ops(), resume);