Re: [Xen-devel] [PATCH] xenpaging:close domU's event channel and free port

[Hongkaixing <hongkaixing@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> Sent: Wednesday, February 15, 2012 5:27 PM
> To: Hongkaixing
> Cc: 'Olaf Hering'; bicky.shi@xxxxxxxxxx; xiaowei.yang@xxxxxxxxxx; 
> xen-devel@xxxxxxxxxxxxxxxxxxx; yanqiangjun@xxxxxxxxxx;
> hanweidong@xxxxxxxxxx
> Subject: RE: [Xen-devel] [PATCH] xenpaging:close domU's event channel and 
> free port
> 
> On Wed, 2012-02-15 at 02:24 +0000, Hongkaixing wrote:
> >
> > > -----Original Message-----
> > > From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> > > Perhaps I'm misunderstanding something and/or showing my ignorance about
> > > how xenpaging works but why does paging need a domU event channel
> > > anyway? Surely paging is transparent to the guest.
> > >
> > > Or is this really a dom0<->Xen event channel which just appears to be
> > > assigned to the guest?
> >
> > In xenpaging source code, there is an inter-domain event channel between 
> > dom0 and domU.
> [...]
> > > Who assigns this remote domain port? Shouldn't it either be closed when
> > > the dom0 end is closed or retained such that it can be reused each time
> > > instead of leaking?
> >
> >   In mem_event_enable(), the function alloc_unbound_xen_event_channel() 
> > allocates a free port for domU,
> > and assigns to xen_consumer;When xenpaging tears down, it just frees dom0's 
> > event channel port by xc_evtchn_unbind(),
> > leaves domU's port still occupied. So we should add the patch to free 
> > domU's port when xenpaging exits.
> 
> The two ends of that event channel are actually dom0 and Xen, because
> chn->xen_consumer is not NULL, even though the Xen end does live in the
> domU evtchn address space. It is not exactly dom0 and domU as you
> suggest, which is where my confused question arose.

   See what xenpaging_init() does when xenpaging is launched:
   xc_mem_paging_enable() ---> this function allocate a event channel of domain 
U, the remote port is stored in paging->mem_event.shared_page->port
            |
            V
    Xc_event_bind_interdomain() --> this function bind dom0 with domU port 
allocated above

   But when xenpaging is tear down:
   xc_mem_paging_disable()  -->  do nothing about event channel
            |
            V
   xc_evtchn_unbind()  -->  free the dom0 port, but leave remote port(domU )  
ECS_UNBOUND


 Hong Kaixing.

> 
> Ian.


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