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Re: [Qemu-devel] Is it possible to detect guest OS modifying pte inside

From: Wei-Ren Chen
Subject: Re: [Qemu-devel] Is it possible to detect guest OS modifying pte inside QEMU?
Date: Wed, 8 Aug 2012 16:33:57 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Aug 08, 2012 at 09:05:18AM +0100, Peter Maydell wrote:
> On 8 August 2012 08:38, 陳韋任 (Wei-Ren Chen) <address@hidden> wrote:
> >   Just for research, we are studying if we can leave the guest page
> > table walk to underlying hardware rather than using software emulation
> > (like current approach). So, maybe (if *doable*) we can use x86 hardware
> > to help us to walk guest (like ARM) page table. The rough idea is we
> > have to maintain a x86-format shadow page table for the corresponding
> > ARM page table, point host cr3 to the shadow page table, and let x86
> > hardware do its job. The problem is, we have to aware that guest is
> > modifying its guest page table entry so that we can sync its corrsponding
> > shadow page table. But, we still haven't find a good way to know when
> > the guest OS is modifying guest page table entry.
> Make it read-only for the guest, and then when you get the exception when
> the guest tries to write it, you can (a) do what you need to do and (b)
> emulate the failing write insn.

  You mean like what KVM does for shadow page table? I think we should
mprotect the QEMU virtual memory which is allocated as a guest page
table by the guest OS, right? If so, how do we know what (QEMU) virtual
memory we should protect? The other way I was thought of is to see if
we can aware some guest code sequence modifying pte, but so far no clue
to it.


Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj

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