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Re: [Xen-devel] Tracking guest pages modified by qemu-dm / backend

To: AP <apxeng@xxxxxxxxx>
Subject: Re: [Xen-devel] Tracking guest pages modified by qemu-dm / backend
From: Shriram Rajagopalan <rshriram@xxxxxxxxx>
Date: Sat, 25 Jun 2011 13:30:31 -0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Tim Deegan <Tim.Deegan@xxxxxxxxxx>
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On Fri, Jun 24, 2011 at 1:22 PM, AP <apxeng@xxxxxxxxx> wrote:
On Fri, Jun 24, 2011 at 1:40 AM, Tim Deegan <Tim.Deegan@xxxxxxxxxx> wrote:
> At 11:06 -0700 on 23 Jun (1308827188), AP wrote:
>> On Wed, Jun 22, 2011 at 1:49 AM, Tim Deegan <Tim.Deegan@xxxxxxxxxx> wrote:
>> > Hi,
>> >
>> > At 15:43 -0700 on 21 Jun (1308671006), AP wrote:
>> >> Consider an HVM domain with EPT. When qemu-dm, backend or any other
>> >> device model modifies DomU memory (like on a disk read or packet
>> >> received), how does Xen track the pages that have been modified? For
>> >> example, when an DomU is being migrated while IO is occurring.
>> >
>> > Backends use grant tables to map guest memory; the granted pages are
>> > marked dirty when the backend disconnects.  Qemu-dm uses the
>> > HVMOP_modified memory hypercall to tell Xen that it's written to
>> > the guest's memory.
>> From what I see for EPT/HAP, log dirty is turned on only during domain
>> save.
> It's also used to track just the virtual framebuffer so qemu can avoid
> having to compute which parts of the screen to send over VNC.
> But for general memory it's only used during save, because there's no
> need for it otherwise.  If you're confused about that maybe you should
> read the original paper on Xen live migration:
> http://www.cl.cam.ac.uk/research/srg/netos/papers/2005-nsdi-migration.pdf

Sorry I think I did not phrase my question well. What I was not able
to understand was how / where the granted pages were being marked
dirty w.r.t EPT after the backend disconnects.

I am not sure what you mean by "backend disconnects", but each time a grant
is unmapped, log dirty bit for the page is set.
Check out the __gnttab_unmap_common function in xen/common/grant_table.c
it calls gnttab_mark_dirty(..) which is a macro for paging_mark_dirty.


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