[Qemu-devel] unique (or otherwise) RAM block names

[Peter Maydell]

I was going through trying to fix up various devices that currently
fail to register RAM blocks for migration, or register the RAM
block globally rather than locally, and I ran into something
unexpected.

We name RAM blocks in qemu_ram_set_idstr() like this:
  * if the user passed a device pointer, then call qdev_get_dev_path(),
    and if that returns non-NULL, use "path/name"
  * otherwise, use "name"

Unfortunately, it turns out that there's no guarantee that
qdev_get_dev_path() will return anything useful. If the device
isn't on a bus, or the bus's class doesn't implement the get_dev_path
method, then it'll return NULL.

In particular, this means that if you create what you expect to
be a local-to-this-device RAM memory region in a SysBusDevice,
then (because SysBus doesn't implement get_dev_path), there is
no per-device qualification added to the region name, and so the
code silently creates a globally-namespaced RAM region.
Trying to create multiple instances of the device therefore fails.

How can we make this work (preferably without breaking migration
compat for existing devices) ?

(Sysbus isn't the only bus that doesn't implement get_dev_path,
it's just the first one I found.)

thanks
-- PMM