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

[Superymk <superymkxen@xxxxxxxxxxx>]

Hi all,

I just implement the EPT support in my hypervisor (Very similar to Newbluepill). My new problem is irrelevant with Xen.
It's just about how to debug VTx implementation in drivers.

Here is the story. First, I implement a driver support partial VTx, and it works very well. Then I implement EPT to identically map gfn to mfn from 0x0 to 0xfffff. I suppose it should be OK. But the result is the Windows OS hangs (No reboot, No BSOD) when executing VMLAUNCH instruction.

And my problem is that, the windbg just shows "debuggee is running" when the debuggee Windows OS hangs, even if I insert "ud2" instruction before the next statement, #VMEXIT handler and the first instruction in non-root mode. VMLAUNCH should not make this happen according to Intel's manual 2B. Everything is OK if I set "enable ept" to be 0 or clear the "EPT pointer" field in VMCS. Can someone explain why this happens and what should I do to continue debugging?

Both the hypervisor and the Windows OS is on x86_32 platform. I use windbg to debug the target machine via serial port.

Some debug information: EPT pointer is 0x9ba801e, (pfn:0x9ba8, flag:0x1e, I have double checked this) PML4[0] = 0x00000000_09cd8007, PDPT[0] = 0x00000000_09cf3007, PD[0] = 0x00000000_09cf2007, PT[0] = 0x00000000_00000077. Other entries of the same scheme with different values.

The debuggee is Intel i5 650, multi-core disabled.

Thanks,
Miao

On 11/17/2010 7:53 PM, Superymk wrote:

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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