[PATCH RESEND] hostmem: Don't report pmem attribute if unsupported

[Michal Privoznik]

When management applications (like Libvirt) want to check whether
memory-backend-file.pmem is supported they can list object
properties using 'qom-list-properties'. However, 'pmem' is
declared always (and thus reported always) and only at runtime
QEMU errors out if it was built without libpmem (and thus can not
guarantee write persistence). This is suboptimal since we have
ability to declare attributes at compile time.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1915216
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
---

This is just a resend of a patch I've sent earlier with Reviewed-by and
Tested-by added:

https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04558.html

 backends/hostmem-file.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 40e1e5b3e3..7e30eb5985 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -123,6 +123,7 @@ static void file_memory_backend_set_align(Object *o, 
Visitor *v,
     fb->align = val;
 }
 
+#ifdef CONFIG_LIBPMEM
 static bool file_memory_backend_get_pmem(Object *o, Error **errp)
 {
     return MEMORY_BACKEND_FILE(o)->is_pmem;
@@ -139,17 +140,9 @@ static void file_memory_backend_set_pmem(Object *o, bool 
value, Error **errp)
         return;
     }
 
-#ifndef CONFIG_LIBPMEM
-    if (value) {
-        error_setg(errp, "Lack of libpmem support while setting the 'pmem=on'"
-                   " of %s. We can't ensure data persistence.",
-                   object_get_typename(o));
-        return;
-    }
-#endif
-
     fb->is_pmem = value;
 }
+#endif /* CONFIG_LIBPMEM */
 
 static void file_backend_unparent(Object *obj)
 {
@@ -180,8 +173,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
         file_memory_backend_get_align,
         file_memory_backend_set_align,
         NULL, NULL);
+#ifdef CONFIG_LIBPMEM
     object_class_property_add_bool(oc, "pmem",
         file_memory_backend_get_pmem, file_memory_backend_set_pmem);
+#endif
 }
 
 static void file_backend_instance_finalize(Object *o)
-- 
2.26.2