Re: [Qemu-devel] [PATCH 00/15] Memory/IOMMU patches part 4: region ownership

[Peter Maydell]

On 2 June 2013 16:43, Paolo Bonzini <address@hidden> wrote:
> Reference counting the region piggybacks on reference counting of a QOM
> object, the "owner" of the region.  The owner API is designed so that
> it will be called as little as possible.  Unowned subregions will get a
> region if memory_region_set_owner is called after the subregion is added.
> This is in general the common case already; often setting the owner can
> be delegated to a bus-specific API that already takes a DeviceState
> (for example pci_register_bar or sysbus_init_mmio).

This feels a bit fragile to me -- there doesn't seem to be
a clear rule for who has to set the owner of a region or
when they have to do it, or for ensuring that it doesn't
get forgotten altogether.

What happens if I take a MemoryRegion* that another device
has exposed to me as a sysbus mmio region (and so claimed
ownership of) and pass it to pci_register_bar()? Who owns
it at that point? [That's a legitimate thing to do, I think,
though I don't suppose anybody does it at the moment.
Sysbus MMIOs aren't only for mapping in the system address
space, they're a general way for one device to expose a
MemoryRegion * for use by another device.]

thanks
-- PMM