x86/p2m: guard altp2m routines

Initialize and bring down altp2m only when it is supported by the platform,
e.g. VMX. Also guard p2m_altp2m_propagate_change().
The purpose of that is the possibility to disable altp2m support and exclude its
code from the build completely, when it's not supported by the target platform.

Here hvm_altp2m_supported() is being used to check for ALTP2M availability,
which is only defined if HVM enabled, so a stub for that routine added for
!HVM configuration as well.

Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/include/asm/hvm/hvm.h

@@ -828,6 +828,11 @@ static inline bool hvm_hap_supported(void)
     return false;
 }
 
+static inline bool hvm_altp2m_supported(void)
+{
+    return false;
+}
+
 static inline bool hvm_nested_virt_supported(void)
 {
     return false;

xen/arch/x86/mm/p2m-basic.c

@@ -128,7 +128,7 @@ int p2m_init(struct domain *d)
         return rc;
     }
 
-    rc = p2m_init_altp2m(d);
+    rc = hvm_altp2m_supported() ? p2m_init_altp2m(d) : 0;
     if ( rc )
     {
         p2m_teardown_hostp2m(d);
@@ -197,11 +197,12 @@ void p2m_final_teardown(struct domain *d)
 {
     if ( is_hvm_domain(d) )
     {
+        if ( hvm_altp2m_supported() )
+            p2m_teardown_altp2m(d);
         /*
-         * We must tear down both of them unconditionally because
-         * we initialise them unconditionally.
+         * We must tear down nestedp2m unconditionally because
+         * we initialise it unconditionally.
          */
-        p2m_teardown_altp2m(d);
         p2m_teardown_nestedp2m(d);
     }
 

xen/arch/x86/mm/p2m-ept.c

@@ -990,7 +990,7 @@ out:
     if ( is_epte_present(&old_entry) )
         ept_free_entry(p2m, &old_entry, target);
 
-    if ( entry_written && p2m_is_hostp2m(p2m) )
+    if ( hvm_altp2m_supported() && entry_written && p2m_is_hostp2m(p2m) )
     {
         ret = p2m_altp2m_propagate_change(d, _gfn(gfn), mfn, order, p2mt, p2ma);
         if ( !rc )