Re: [Qemu-devel] [PATCH qemu v7 06/14] spapr_iommu: Introduce "enabled" state for TCE table

[Paolo Bonzini <pbonzini@redhat.com>]

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On 26/05/2015 04:46, David Gibson wrote:
> On Tue, May 26, 2015 at 01:05:56AM +1000, Alexey Kardashevskiy 
> wrote:
>> Hi Paolo,
>> 
>> I have had a conversation with Mike and it turns out I am not 
>> allowed to create/remove memory regions dynamically 
>> (docs/memory.txt:101); otherwise "destroying regions during
>> reset causes assertion in RCU thread during PHB/IOMMU
>> unplug/unparent". Is it because patch just missing some
>> unref()/unparent() call or it is totally wrong and I have to
>> implement subregions (on a PCI bus address space) myself if I
>> want dynamic DMA windows? Thanks!

I'm blind, can you explain the path where that happens?

> So, the sentences after that one note an exception for alias and 
> container regions.  I think iommu regions should behave similarly
> - in a sense they're just a procedurally generated collection of 
> alias regions.

The difference is that containers and aliases are resolved at the time
the memory region tree is flattened, while IOMMU regions are resolved
at run time.

> If it's not true now that they can be unparented at any time like 
> alias regions, we should probably try to make it true.

Unfortunately it's not so easy...

Paolo