Add host_memory_backend_pagesize() helper

There are a couple places (one generic, one target specific) where we need
to get the host page size associated with a particular memory backend.  I
have some upcoming code which will add another place which wants this.  So,
for convenience, add a helper function to calculate this.

host_memory_backend_pagesize() returns the host pagesize for a given
HostMemoryBackend object.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

backends/hostmem.c

@@ -18,6 +18,7 @@
 #include "qapi/visitor.h"
 #include "qemu/config-file.h"
 #include "qom/object_interfaces.h"
+#include "qemu/mmap-alloc.h"
 
 #ifdef CONFIG_NUMA
 #include <numaif.h>
@@ -262,6 +263,23 @@ bool host_memory_backend_is_mapped(HostMemoryBackend *backend)
     return backend->is_mapped;
 }
 
+#ifdef __linux__
+size_t host_memory_backend_pagesize(HostMemoryBackend *memdev)
+{
+    Object *obj = OBJECT(memdev);
+    char *path = object_property_get_str(obj, "mem-path", NULL);
+    size_t pagesize = qemu_mempath_getpagesize(path);
+
+    g_free(path);
+    return pagesize;
+}
+#else
+size_t host_memory_backend_pagesize(HostMemoryBackend *memdev)
+{
+    return getpagesize();
+}
+#endif
+
 static void
 host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
 {