Re: [PATCH 3/5] vhost: support upto 509 memory regions

Paolo Bonzini
2015-06-19T10:44:39Z

On 19/06/2015 12:14, Michael S. Tsirkin wrote:
> On Fri, Jun 19, 2015 at 10:52:47AM +0200, Paolo Bonzini wrote:
>>
>>
>> On 19/06/2015 10:05, Michael S. Tsirkin wrote:
>>>> No, only destruction of the memory region frees it. address_space_map
>>>> takes a reference to the memory region and address_space_unmap releases it.
>>>>
>>>> Paolo
>>>
>>> Confused. So can we call mmap(MAP_NORESERVE) in address_space_unmap
>>> after we detect refcount is 0?
>>
>> No, because in the meanwhile another DIMM could have been hotplugged
>> at the same place where the old one was. This is legal:
>>
>> user guest QEMU
>> ----------------------------------------------------------------------------------------
>> start I/O
>> '---------------> address_space_map
>> device_del
>> '-------------------> receives SCI
>> executes _EJ0
>> '---------------> memory_region_del_subregion
>> object_unparent
>
> So guest started DMA into memory, then ejected this memory while DMA
> is in progress?

Yes. There is nothing that forbids doing that.

Paolo

>> device_add
>> '-----------------------------------------> device_set_realized
>> hotplug_handler_plug
>> pc_machine_device_plug_cb
>> pc_dimm_plug
>> memory_region_add_subregion
>>
>> I/O finishes
>> address_space_unmap
>>
>> Surprise removal similarly could be done in QEMU, but it will hold to
>> some resources for as long as the device backends need them.
>>
>> Paolo