linux-kernel - Re: [kvm-devel] [PATCH] export notifier #1

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Date:	Wed, 23 Jan 2008 15:12:36 +0100
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Robin Holt <holt@....com>
CC:	Christoph Lameter <clameter@....com>,
	Andrea Arcangeli <andrea@...ranet.com>,
	Andrew Morton <akpm@...l.org>, Nick Piggin <npiggin@...e.de>,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	steiner@....com, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [kvm-devel] [PATCH] export notifier #1

  Hi,

>> That would render the notifies useless for Xen too.  Xen needs to
>> intercept the actual pte clear and instead of just zapping it use the
>> hypercall to do the unmap and release the grant.
> 
> We are tackling that by having our own page table hanging off the
> structure representing our seg (thing created when we do the equiv of
> your grant call).

--verbose please.  I don't understand that "own page table" trick.  Is
that page table actually used by the processor or is it just used to
maintain some sort of page list?

>> Current implementation uses a new vm_ops operation which is called if
>> present instead of doing a ptep_get_and_clear_full().  It is in the
>> XenSource tree only, mainline hasn't this yet due to implementing only
>> the DomU bits so far.  When adding Dom0 support to mainline we'll need
>> some way to handle it, and I'd like to see the notifies be designed in a
>> way that Xen can simply use them.
> 
> Would the callouts Christoph proposed work for you if you maintained
> your own page table and moved them after the callouts the mmu_notifiers
> are using.

I *think* it would.  I'm not that deep in the VM details to be sure
though.  One possible problem I see is that the hypercall does also tear
down the mapping, so this isn't just a notify but also changes the page
tables, which could confuse the VM later on when it comes to the actual
pte clearing.

cheers,
  Gerd

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