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

[Michael S. Tsirkin]

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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