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: Friday, February 17, 2012 4:21 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 Fri, 2012-02-17 at 06:21 +0000, Hongkaixing wrote:
> >
> > > -----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
> 
> I wasn't querying the presence of the leak or the validity of the fix, I
> was asking what the domU event channel was even for (i.e. why does it
> exist at all?).

Sorry! The event channel is used to wake domU up when its page has already been 
paged in.

> 
> I answered that myself in my previous mail -- it's actually a Xen event
> channel not a guest visible one.
> 
> Ian.
> 
> >
> >
> >  Hong Kaixing.
> >
> > >
> > > Ian.
> >



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