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

Michael S. Tsirkin
2015-06-19T13:35:05Z

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.
>
> Paolo

Can we simply defer the next device_add using a hva until all IO completes?

> >> 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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