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

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Date:	Fri, 19 Jun 2015 15:34:52 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...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 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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