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Re: [Xen-devel] NPT and EPT question

from [Tim Deegan]

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To:	bo ma <kyle.ma83@xxxxxxxxx>
Subject:	Re: [Xen-devel] NPT and EPT question
From:	Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date:	Mon, 17 Nov 2008 13:41:23 +0000
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Hi,

At 21:23 +0800 on 17 Nov (1226957017), bo ma wrote:
> Recent I read AMD specification about NPT,and met with a question about a
> sentence.
> 
> The sentence is : Note that host table walks for guest page tables are
> always treated as data writes.I don't know why treated as data writes?

Possibly because the guest-table walker might have to set the A and D
bits, and the hardware doesn't know whether it will do that until
_after_ it's done the host-table walk.

Maybe someone from AMD can clarify. 

> In intel EPT ,is it the same? or treated as data reads?

My impression is that EPT only requires write permissions if it actually
writes back an A or D bit, but I'm not sure.   It should be easy to check.

Cheers,

Tim

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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