x86: introduce using_{svm,vmx}() helpers

As we now have AMD_SVM/INTEL_VMX config options for enabling/disabling these
features completely in the build, we need some build-time checks to ensure that
vmx/svm code can be used and things compile. Macros cpu_has_{svm,vmx} used to be
doing such checks at runtime, however they do not check if SVM/VMX support is
enabled in the build.

Also cpu_has_{svm,vmx} can potentially be called from non-{VMX,SVM} build
yet running on {VMX,SVM}-enabled CPU, so would correctly indicate that VMX/SVM
is indeed supported by CPU, but code to drive it can't be used.

New routines using_{vmx,svm}() indicate that both CPU _and_ build provide
corresponding technology support, while cpu_has_{vmx,svm} still remains for
informational runtime purpose, just as their naming suggests.

These new helpers are used right away in several sites, namely guard calls to
start_nested_{svm,vmx} and start_{svm,vmx} to fix a build when INTEL_VMX=n or
AMD_SVM=n.

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

xen/arch/x86/hvm/hvm.c

@@ -155,9 +155,9 @@ static int __init cf_check hvm_enable(void)
 {
     const struct hvm_function_table *fns = NULL;
 
-    if ( cpu_has_vmx )
+    if ( using_vmx() )
         fns = start_vmx();
-    else if ( cpu_has_svm )
+    else if ( using_svm() )
         fns = start_svm();
 
     if ( fns == NULL )

xen/arch/x86/hvm/nestedhvm.c

@@ -155,9 +155,9 @@ static int __init cf_check nestedhvm_setup(void)
      * done, so that if (for example) HAP is disabled, nested virt is
      * disabled as well.
      */
-    if ( cpu_has_vmx )
+    if ( using_vmx() )
         start_nested_vmx(&hvm_funcs);
-    else if ( cpu_has_svm )
+    else if ( using_svm() )
         start_nested_svm(&hvm_funcs);
 
     return 0;

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

@@ -363,6 +363,16 @@ int hvm_copy_context_and_params(struct domain *dst, struct domain *src);
 
 int hvm_get_param(struct domain *d, uint32_t index, uint64_t *value);
 
+static inline bool using_vmx(void)
+{
+    return IS_ENABLED(CONFIG_INTEL_VMX) && cpu_has_vmx;
+}
+
+static inline bool using_svm(void)
+{
+    return IS_ENABLED(CONFIG_AMD_SVM) && cpu_has_svm;
+}
+
 #ifdef CONFIG_HVM
 
 #define hvm_get_guest_tsc(v) hvm_get_guest_tsc_fixed(v, 0)