Re: [Xen-devel] Understanding AMD NPT in xen

["Huang2, Wei" <Wei.Huang2@xxxxxxx>]

White page info is more accurate. The slides was a bit vague. Here are the 
steps, assuming that CPU doesn't have TLB:

1. To find out gPML4E
* Traverse nested page table using gCR3 to locate guest level-4 page (PML4 
table)
* Use PML4 offset (bit 47:39) to get the value of gPML4E
2. To find out gPDPE
* Traverse nested page table using gPML4E to locate guest level-3 page (PDP 
table)
* Use PDP offset (bit 38:30) to get the value of gPDPE
3. To find out gPDE
* Traverse nested page table using gPDPE to locate guest level-2 page (PD table)
* Use PD offset (bit 29:21) to get the value of gPDE
4. To find out gPTE
* Traverse nested page table using gPDE to locate guest level-1 page (PT table)
* Use PT offset (bit 20:12) to get the value of gPTE
5. To find out gData
* Traverse nested page table using gPTE to locate guest data page
* Use physical page offset (bit 11:0) to get the data value

TLB will accelerate this walking significantly. 

-Wei

-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-bounces@xxxxxxxxxxxxx] 
On Behalf Of Steven
Sent: Thursday, April 12, 2012 12:06 PM
To: Wang2, Wei; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Understanding AMD NPT in xen

Hi, Wei,
I have read you slides in xen summit 2007 about NPT in AMD, "AMD
Barcelona and Nested Paging Support in Xen".
However, I have some question regarding the nested page walk in your slide 8.

In your slide 8, the first step is to get gPA from get_PML4(gCR3,gVA).
I assume that it use the [47:39] bit of gVA.
However, in another AMD white paper, "AMD-VTM Nested Paging".
http://developer.amd.com/assets/NPT-WP-1%201-final-TM.pdf
In its figure 4, I saw that the first step is to translate gCR3 using
nested page walk and then combine with the gVA[47:39] to read the
table entry.

These two documents look having different order of reading the guest
page table. In the slides, it first get_PML4. But in the white paper,
it first does nested page walk.
I am wondering which one is true. Thanks.

- ha

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