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

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Date:	Fri, 19 Jun 2015 12:44:26 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Igor Mammedov <imammedo@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, andrey@...l.ru
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions



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
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