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Re: [Xen-devel] shadow OOS and fast path are incompatible

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] shadow OOS and fast path are incompatible
From: Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>
Date: Fri, 3 Jul 2009 11:20:21 +0200
Cc: Frank van der Linden <Frank.Vanderlinden@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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About the read-only MMIO,

On Fri, Jul 3, 2009 at 10:50 AM, Tim Deegan<Tim.Deegan@xxxxxxxxxx> wrote:
> Yes.  The fast-not-present path is safe without OOS because the changing
> L1 would have to be as a result of the guest writing it, which is a race
> on real hardware.   The fast-MMIO path is OK even with OOS because it
> only caches present entries, but for proper safety we might want to
> avoid using fast-path MMIO if the guest maps MMIO space read-only (does
> anyone actually do that?)

That is still correct. The fast path gets only the gfn + offset and
pass it to the mmio handling code, that will check the permission by
using proper locks.


It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
                                                  E. W. Dijkstra

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