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Re: [Xen-devel] How EPT translates an X86_32 guest physical address?

To: Superymk <superymkxen@xxxxxxxxxxx>
Subject: Re: [Xen-devel] How EPT translates an X86_32 guest physical address?
From: Chu Rui <ruichu@xxxxxxxxx>
Date: Wed, 17 Nov 2010 19:23:40 +0800
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>
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Also thanks to George!  I am just reading p2m-ept.c three hours ago.  As a newbie, it is a hard work for me... wish I could get your kind help later.
2010/11/17 Superymk <superymkxen@xxxxxxxxxxx>
Thanks a lot, George. Also thanks for Chu's material.

On 11/17/2010 6:32 PM, George Dunlap wrote:
The exact implementation of 32-bit mode on a 64-bit capable processor is something only the engineers at Intel know; but logically yes, whatever it does is equivalent to first zero-extending the 32-bit value.

You can see the software implementation in xen/arch/x86/mm/hap/p2m-ept.c:ept_get_entry().  That function is passed an unsigned long, which in 64-bit mode is 64 bits, so at that point any hardware address would have been zero-extended.


On 17/11/10 10:20, Superymk wrote:
So your point is the guest CR3 needs to be "extended" to 64 bits with
zeroes first, if it is a 32-bit guest. right?

On 11/17/2010 6:11 PM, George Dunlap wrote:
If you're in 64-bit mode and the hardware had a TLB miss for virtual
address of 0xdeadb000, how would the hardware walk the pagetables?
There are 20 bits for the virtual frame number, but each page-table
entry has 9 bits.

It's the exact same situation if the guest cr3 was set to 0xdeadb000.
The indexes into the higher-level tables would simply be zero.


On Wed, Nov 17, 2010 at 9:40 AM, Superymk<superymkxen@xxxxxxxxxxx>   wrote:
Your figure points out the exactly EPT translation mechanism for an X64

In the face of an X86_32 guest, how can EPT find the right EPML4 entry when
translating CR3's pfn value into the right mfn value? There are 20 bits for
indexing in total, while each level of EPT paging structure uses only 9 bits
for indexing.

On 11/17/2010 5:20 PM, Chu Rui wrote:

Maybe this figure depicts the process...

The original URL is http://software.intel.com/file/25040

2010/11/17 Superymk<superymkxen@xxxxxxxxxxx>
Hi all,

Can some one please tell me how EPT translates an X86_32 guest physical
address? I have read the Intel's manual, but it seems there is no discussion
about this condition.

My concern is that, the guest CR3 pfn can be considered as being
constituted by two 10 bits indexers for an X86_32 virtual machine. However,
the EPT paging structures is similar with the page tables used on X86_64
platform. which has four 9 bits indexers in its address layout. In addition,
each EPT entry is 64 bits long. Hence, a 4K page can hold at most 512
entries. So, if the guest CR3's pfn is 0xfffff (an X86_32 virtual machine)
and I get a valid EPTP, how EPT will perform the translation?


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