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Re: [Xen-devel] write page table in user mode

  • To: "Tim Deegan" <Tim.Deegan@xxxxxxxxxx>
  • From: "Michael A Fetterman" <Michael.Fetterman@xxxxxxxxxxxx>
  • Date: Mon, 4 Feb 2008 12:02:09 +0000
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Su, Disheng" <disheng.su@xxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>
  • Delivery-date: Mon, 04 Feb 2008 04:02:36 -0800
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> > > It's said to be a forward progress issue, that instruction page of 
> > > faulting
> > > IP falls into mapped virtual range by same L1 as the target frame it tries
> > > to update. So the implication is that the unshadow unfortunately
> > > succeeds.
> Do you mean that the guest instruction stream is changing the VM mapping
> of the page under %eip, from user-space?  Wow.

When I first read Kevin's description, I took it to mean that the same L1 *page*
(not entry) was mapping the current EIP and the target of the write.  If this is
case, then the current code's prefetching of L1 entries on the same page is
probably interacting with the user-space write heuristic, causing the
shadow->unshadow->shadow livelock, making for a much easier fix.

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