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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-02-13 at 05:47 +0000, Hongkaixing wrote:
> 
> > -----Original Message-----
> > From: Olaf Hering [mailto:olaf@xxxxxxxxx]
> > Sent: Saturday, February 11, 2012 12:40 AM
> > To: hongkaixing@xxxxxxxxxx
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; yanqiangjun@xxxxxxxxxx; 
> > bicky.shi@xxxxxxxxxx; xiaowei.yang@xxxxxxxxxx;
> > hanweidong@xxxxxxxxxx
> > Subject: Re: [PATCH] xenpaging:close domU's event channel and free port
> > 
> > On Thu, Feb 09, hongkaixing@xxxxxxxxxx wrote:
> > 
> > > xenpaging:close domU's event channel and free port
> > >
> > > Every domain (X86 64 bit)has 4096 event channels.In source code,
> > > domU's event channel is allocated in mem_event_enable(),but just
> > > unbind dom0's event channel in xenpaging_teardown().This bug will
> > > result in that we can not use xenpaging after reopening it for 4096
> > > times.We should free domU's event channel in mem_event_disable().so
> > > that we can reuse the port.
> > 
> > Does that fix a real bug?
> > 
> > xenpaging_teardown() does both xc_mem_paging_disable() and
> > xc_evtchn_unbind(). The former fails often because the domain is gone
> > and so it doesnt even reach the function in mem_event.c.
> > The latter is called unconditionally.
> 
>    I have tested whether the kernel driver does a cleanup of all used ports 
> once xenpaging exits.
> Every domain has 1024 event channels.In xc_evtchn_unbind(),it just frees 
> dom0's event channel's port,
> and changes its state to be ECS_FREE;but the remote domain(domU)'s port is 
> still ECS_UNBOUND.

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?

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?

> so when each domU triggers 1024 times of xenpaging,it will fail of "Error 
> initialising shared page
> (28 = No space left on device): Internal error".Because there is no available 
> free port for this domU.
> Only the port's state is ECS_FREE,then it can be allocated by get_free_port();
> 
> 
> > 
> > Also I would expect that once xenpaging exits the kernel driver does a
> > cleanup of all used ports. I havent checked wether thats true.
> > 
> > Olaf
> 
> 
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