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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> 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.

For device we also have a nmumerical 'instance_id' parameter that
some odd setups use.

> 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.)

My preference would be that for new machine types we should
define get_dev_path on the busses that are missing it.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK