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

Michael S. Tsirkin
2015-06-19T10:15:05Z

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?

> 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