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

[Superymk <superymkxen@xxxxxxxxxxx>]

Thanks for Ian's answer. it comes to a more general scenario.

Hi Chu, EPT entry is 64 bit long, regardless the hypervisor is on x86_32 platform or x86_64 platform. So there is no difference for the hypervisor to use EPT on these two platforms.

On 11/17/2010 7:26 PM, Chu Rui wrote:

Okay, in my mind, the hardware has only one work mode, 32bit or 64bit. Thus the 32bit guest address will be extended under the 64bit host.

But what will happen for a 64bit guest under a 32bit host :-)

2010/11/17 Ian Campbell <Ian.Campbell@xxxxxxxxxx>

On Wed, 2010-11-17 at 10:32 +0000, 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.

Even on x86_32 physical addresses are >32 bit (think PAE). cr3 is a
physical address, even if the register which exposes it happens to be
limited to 32 bits. cr3 has probably already been expanded to a full
physical address by the time EPT sees it and I don't think there's any
difference between 32 and 64 bit (at least in this aspect) in how EPT
handles the translation from physical address to machine address.

Ian.

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