[PATCH 2/3] hostmem-file: add readonly=on|off option

[Stefan Hajnoczi <stefanha@redhat.com>]

Let -object memory-backend-file work on read-only files when the
readonly=on option is given. This can be used to share the contents of a
file between multiple guests while preventing them from consuming
Copy-on-Write memory if guests dirty the pages, for example.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 backends/hostmem-file.c | 26 +++++++++++++++++++++++++-
 qemu-options.hx         |  5 ++++-
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 37c70acfe2..6bd5bf9b91 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -30,6 +30,7 @@ struct HostMemoryBackendFile {
     uint64_t align;
     bool discard_data;
     bool is_pmem;
+    bool readonly;
 };
 
 static void
@@ -57,7 +58,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                      backend->size, fb->align,
                                      (backend->share ? RAM_SHARED : 0) |
                                      (fb->is_pmem ? RAM_PMEM : 0),
-                                     fb->mem_path, false, errp);
+                                     fb->mem_path, fb->readonly, errp);
     g_free(name);
 #endif
 }
@@ -152,6 +153,26 @@ static void file_memory_backend_set_pmem(Object *o, bool value, Error **errp)
     fb->is_pmem = value;
 }
 
+static bool file_memory_backend_get_readonly(Object *o, Error **errp)
+{
+    return MEMORY_BACKEND_FILE(o)->readonly;
+}
+
+static void file_memory_backend_set_readonly(Object *o, bool value,
+                                             Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property 'readonly' of %s.",
+                   object_get_typename(o));
+        return;
+    }
+
+    fb->readonly = value;
+}
+
 static void file_backend_unparent(Object *obj)
 {
     HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -183,6 +204,9 @@ file_backend_class_init(ObjectClass *oc, void *data)
         NULL, NULL);
     object_class_property_add_bool(oc, "pmem",
         file_memory_backend_get_pmem, file_memory_backend_set_pmem);
+    object_class_property_add_bool(oc, "readonly",
+        file_memory_backend_get_readonly,
+        file_memory_backend_set_readonly);
 }
 
 static void file_backend_instance_finalize(Object *o)
diff --git a/qemu-options.hx b/qemu-options.hx
index 708583b4ce..d834e00b0d 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4369,7 +4369,7 @@ SRST
     they are specified. Note that the 'id' property must be set. These
     objects are placed in the '/objects' path.
 
-    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align``
+    ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,readonly=on|off``
         Creates a memory file backend object, which can be used to back
         the guest RAM with huge pages.
 
@@ -4452,6 +4452,9 @@ SRST
         4.15) and the filesystem of ``mem-path`` mounted with DAX
         option.
 
+        The ``readonly`` option specifies whether the backing file is opened
+        read-only or read-write (default).
+
     ``-object memory-backend-ram,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
         Creates a memory backend object, which can be used to back the
         guest RAM. Memory backend objects offer more control than the
-- 
2.26.2