It's easy to miss that memory-backend-file with "share=off" (default)
will always try opening the file R/W as default, and fail if we don't
have write permissions to the file.

In that case, the user has to explicit specify "readonly=on,rom=off" to
get usable RAM, for example, for VM templating.

Let's hint that '-object memory-backend-file,readonly=on,rom=off,...'
exists to consume R/O files in a private mapping to create writable RAM,
but only if we have permissions to open the file read-only.

Suggested-by: ThinerLogoer <logoerthiner1@163.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 softmmu/physmem.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 2d903e479b..d812276f41 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1970,6 +1970,25 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
     if (fd < 0) {
         error_setg_errno(errp, -fd, "can't open backing store %s for guest RAM",
                          mem_path);
+        if (!(ram_flags & RAM_READONLY_FD) && !(ram_flags & RAM_SHARED) &&
+            fd == -EACCES) {
+            /*
+             * If we can open the file R/O (note: will never create a new file)
+             * and we are dealing with a private mapping, there are still ways
+             * to consume such files and get RAM instead of ROM.
+             */
+            fd = file_ram_open(mem_path, memory_region_name(mr), true,
+                               &created);
+            if (fd < 0) {
+                return NULL;
+            }
+            assert(!created);
+            close(fd);
+            error_append_hint(errp, "Consider opening the backing store"
+                " read-only but still creating writable RAM using"
+                " '-object memory-backend-file,readonly=on,rom=off...'"
+                " (see \"VM templating\" documentation)\n");
+        }
         return NULL;
     }
 
-- 
2.41.0