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

Michael S. Tsirkin
2015-06-19T16:20:41Z

On Fri, Jun 19, 2015 at 05:19:44PM +0200, Paolo Bonzini wrote:
>
>
> 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

No need to audit them all: defer device_add using an hva range until
address_space_unmap drops using hvas in range drops reference count to
0.

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