VMLAUNCH "hangs" when enabling EPT

VMLAUNCH "hangs" when enabling EPT

Hi all,

I just implement the EPT support in my hypervisor (Very similar to
Newbluepill). My problem is how to debug VTx implementation in drivers.

Here is the story. First, I implement a driver to 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

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 are of the same scheme with different
I noticed that Xen 3.4.2 has the same flag in EPT pointer field.

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



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Best Reply

Hi Miao,

Looks like Haitao Shan has answered your question at


"One difference between EPT-backed 1:1 mapping and direct 1:1 mapping (I suppose that you don't use shadow page table) is that MTRRs are ignored when EPT is in effect. Do you set up EPT memory type correctly? Especially Windows is likely to access MMIO space below 1M."

Appears to be an issue with memory type settings in your EPT entries.

David Ott

Sorry for the late reply. Yes, the root problem is I filled the entries with the wrong memory type. Now it works. Thank you all the same. :-)

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