Re: [Xen-devel] RE: EPT walks for guest page table

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi,

At 09:10 -0800 on 24 Nov (1227517847), Nakajima, Jun wrote:
> Even if the guest wants to read data or execute code, the guest page
> table may need to be modified for the processor to set the A bit when
> running with paging enabled. This means NPT or EPT needs to set the W
> permission for the guest page table pages to avoid VM exits or EPT
> violation. Depending on the implementation, the A bit may need to be
> set upfront even if the translation turns to be invalid.

I believe the question is whether, if all the A and D bits were already
set, an EPT page walk would still fault if it did not have write access
to the guest pagetable page.

NPT documentation explicitly says that _all_ accesses made by the
guest-pagetable walker are treated as writes, whether they actually
write back to the guest pagetable or not.

The EPT docs talk about A/D-bit-setting being treated as a read-modify-write
operation but don't say what happens if the bit is already set.

Cheers,

Tim.

> Thanks,
> Jun
> ---
> Intel Open Source Technology Center
> From: bo ma [mailto:kyle.ma83@xxxxxxxxx]
> Sent: Monday, November 24, 2008 1:37 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: Nakajima, Jun; Xin, Xiaohui
> Subject: EPT walks for guest page table
> 
> 
> Hi,
> 
> I am really confused about  NPT walks for guest page table , and  eager to 
> know the details about it.
> It is  treated as data writes even if the access itself is a code read in AMD 
> NPT.I just want to know : is it the same in EPT?
> 
> Could anyone help me? I would like to know that very much
> Thank you in advance
> 
> Bo Ma

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

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