accel/xen: Fix xen_enabled() behavior on target-agnostic objects

CONFIG_XEN is generated by configure and stored in "config-target.h",
which is (obviously) only include for target-specific objects.
This is a problem for target-agnostic objects as CONFIG_XEN is never
defined and xen_enabled() is always inlined as 'false'.

Fix by following the KVM schema, defining CONFIG_XEN_IS_POSSIBLE
when we don't know to force the call of the non-inlined function,
returning the xen_allowed boolean.

Fixes: da278d58 ("accel: Move Xen accelerator code under accel/xen/")
Reported-by: Paul Durrant <pdurrant@amazon.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200804074930.13104-2-philmd@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

accel/stubs/xen-stub.c

@@ -9,6 +9,8 @@
 #include "hw/xen/xen.h"
 #include "qapi/qapi-commands-misc.h"
 
+bool xen_allowed;
+
 void xenstore_store_pv_console_info(int i, Chardev *chr)
 {
 }

accel/xen/xen-all.c

@@ -32,12 +32,7 @@
     do { } while (0)
 #endif
 
-static bool xen_allowed;
-
-bool xen_enabled(void)
-{
-    return xen_allowed;
-}
+bool xen_allowed;
 
 xc_interface *xen_xc;
 xenforeignmemory_handle *xen_fmem;

include/sysemu/xen.h

@@ -8,9 +8,19 @@
 #ifndef SYSEMU_XEN_H
 #define SYSEMU_XEN_H
 
-#ifdef CONFIG_XEN
+#ifdef NEED_CPU_H
+# ifdef CONFIG_XEN
+#  define CONFIG_XEN_IS_POSSIBLE
+# endif
+#else
+# define CONFIG_XEN_IS_POSSIBLE
+#endif
+
+#ifdef CONFIG_XEN_IS_POSSIBLE
+
+extern bool xen_allowed;
 
-bool xen_enabled(void);
+#define xen_enabled()           (xen_allowed)
 
 #ifndef CONFIG_USER_ONLY
 void xen_hvm_modified_memory(ram_addr_t start, ram_addr_t length);
@@ -18,7 +28,7 @@ void xen_ram_alloc(ram_addr_t ram_addr, ram_addr_t size,
                    struct MemoryRegion *mr, Error **errp);
 #endif
 
-#else /* !CONFIG_XEN */
+#else /* !CONFIG_XEN_IS_POSSIBLE */
 
 #define xen_enabled() 0
 #ifndef CONFIG_USER_ONLY
@@ -33,6 +43,6 @@ static inline void xen_ram_alloc(ram_addr_t ram_addr, ram_addr_t size,
 }
 #endif
 
-#endif /* CONFIG_XEN */
+#endif /* CONFIG_XEN_IS_POSSIBLE */
 
 #endif