For the most cases arch_vcpu_ioreq_completion() routine is just an empty stub,
except when handling VIO_realmode_completion, which only happens on HVM
domains running on VT-x machine. When VT-x is disabled in build configuration,
both x86 & arm version of routine become empty stubs.
To dispose of these useless stubs we can do optional call to arch-specific
ioreq completion handler, if it's present, and drop arm and generic x86 handlers.
Actual handling of VIO_realmore_completion can be done by VMX code then.

Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
---
 xen/arch/arm/ioreq.c       |  6 ------
 xen/arch/x86/hvm/ioreq.c   | 25 -------------------------
 xen/arch/x86/hvm/vmx/vmx.c | 15 +++++++++++++++
 xen/common/ioreq.c         |  5 ++++-
 xen/include/xen/ioreq.h    |  2 +-
 5 files changed, 20 insertions(+), 33 deletions(-)

diff --git a/xen/arch/arm/ioreq.c b/xen/arch/arm/ioreq.c
index 5df755b48b..2e829d2e7f 100644
--- a/xen/arch/arm/ioreq.c
+++ b/xen/arch/arm/ioreq.c
@@ -135,12 +135,6 @@ bool arch_ioreq_complete_mmio(void)
     return false;
 }
 
-bool arch_vcpu_ioreq_completion(enum vio_completion completion)
-{
-    ASSERT_UNREACHABLE();
-    return true;
-}
-
 /*
  * The "legacy" mechanism of mapping magic pages for the IOREQ servers
  * is x86 specific, so the following hooks don't need to be implemented on Arm:
diff --git a/xen/arch/x86/hvm/ioreq.c b/xen/arch/x86/hvm/ioreq.c
index b37bbd660b..088650e007 100644
--- a/xen/arch/x86/hvm/ioreq.c
+++ b/xen/arch/x86/hvm/ioreq.c
@@ -29,31 +29,6 @@ bool arch_ioreq_complete_mmio(void)
     return handle_mmio();
 }
 
-bool arch_vcpu_ioreq_completion(enum vio_completion completion)
-{
-    switch ( completion )
-    {
-#ifdef CONFIG_VMX
-    case VIO_realmode_completion:
-    {
-        struct hvm_emulate_ctxt ctxt;
-
-        hvm_emulate_init_once(&ctxt, NULL, guest_cpu_user_regs());
-        vmx_realmode_emulate_one(&ctxt);
-        hvm_emulate_writeback(&ctxt);
-
-        break;
-    }
-#endif
-
-    default:
-        ASSERT_UNREACHABLE();
-        break;
-    }
-
-    return true;
-}
-
 static gfn_t hvm_alloc_legacy_ioreq_gfn(struct ioreq_server *s)
 {
     struct domain *d = s->target;
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 8ba996546f..4f9be50fe7 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2749,6 +2749,20 @@ static void cf_check vmx_set_reg(struct vcpu *v, unsigned int reg, uint64_t val)
     vmx_vmcs_exit(v);
 }
 
+bool realmode_vcpu_ioreq_completion(enum vio_completion completion)
+{
+    struct hvm_emulate_ctxt ctxt;
+
+    if ( completion != VIO_realmode_completion )
+        ASSERT_UNREACHABLE();
+
+    hvm_emulate_init_once(&ctxt, NULL, guest_cpu_user_regs());
+    vmx_realmode_emulate_one(&ctxt);
+    hvm_emulate_writeback(&ctxt);
+
+    return true;
+}
+
 static struct hvm_function_table __initdata_cf_clobber vmx_function_table = {
     .name                 = "VMX",
     .cpu_up_prepare       = vmx_cpu_up_prepare,
@@ -3070,6 +3084,7 @@ const struct hvm_function_table * __init start_vmx(void)
     lbr_tsx_fixup_check();
     ler_to_fixup_check();
 
+    arch_vcpu_ioreq_completion = realmode_vcpu_ioreq_completion;
     return &vmx_function_table;
 }
 
diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
index 1257a3d972..94fde97ece 100644
--- a/xen/common/ioreq.c
+++ b/xen/common/ioreq.c
@@ -33,6 +33,8 @@
 #include <public/hvm/ioreq.h>
 #include <public/hvm/params.h>
 
+bool (*arch_vcpu_ioreq_completion)(enum vio_completion completion) = NULL;
+
 void ioreq_request_mapcache_invalidate(const struct domain *d)
 {
     struct vcpu *v = current;
@@ -244,7 +246,8 @@ bool vcpu_ioreq_handle_completion(struct vcpu *v)
         break;
 
     default:
-        res = arch_vcpu_ioreq_completion(completion);
+        if ( arch_vcpu_ioreq_completion )
+            res = arch_vcpu_ioreq_completion(completion);
         break;
     }
 
diff --git a/xen/include/xen/ioreq.h b/xen/include/xen/ioreq.h
index cd399adf17..880214ec41 100644
--- a/xen/include/xen/ioreq.h
+++ b/xen/include/xen/ioreq.h
@@ -111,7 +111,7 @@ void ioreq_domain_init(struct domain *d);
 int ioreq_server_dm_op(struct xen_dm_op *op, struct domain *d, bool *const_op);
 
 bool arch_ioreq_complete_mmio(void);
-bool arch_vcpu_ioreq_completion(enum vio_completion completion);
+extern bool (*arch_vcpu_ioreq_completion)(enum vio_completion completion);
 int arch_ioreq_server_map_pages(struct ioreq_server *s);
 void arch_ioreq_server_unmap_pages(struct ioreq_server *s);
 void arch_ioreq_server_enable(struct ioreq_server *s);
-- 
2.25.1