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From: Laszlo Ersek <lersek@redhat.com>
Message-ID: <3e70445d-dc3c-0cf0-af56-934600f0511d@redhat.com>
Date: Mon, 4 Jun 2018 14:48:21 +0200
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Subject: Re: [Qemu-devel] unique (or otherwise) RAM block names
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To: Peter Maydell <peter.maydell@linaro.org>, QEMU Developers <qemu-devel@nongnu.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>

On 06/01/18 19:34, Peter Maydell 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.
> 
> 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.)

Related: commit f58b39d2d5b6 ("virtio-mmio: format transport base
address in BusClass.get_dev_path", 2016-07-14).

Thanks
Laszlo