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

[Paolo Bonzini]

On 19/06/2015 15:34, Michael S. Tsirkin wrote:
> On Fri, Jun 19, 2015 at 12:44:26PM +0200, Paolo Bonzini wrote:
>>
>>
>> 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.
> 
> Can we simply defer the next device_add using a hva until all IO completes?

We could, but I/O is just an example.  It can be I/O, a network ring,
whatever.  We cannot audit all address_space_map uses.

Paolo
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