Monday, October 19, 2009


A boundary case observed on Intel Merom:

(a) The virtual-machine is configured to vmexit on INVLPG(bit 9 of the PROCESSOR_EXECUTION_CONTROLS is 1).

(b) The virtual-machine has GS BASE = 0xFFFF8000_00000000

(c) Virtual machine executes: invlpg [gs:0-1]

(d) Execution of invlpg causes vmexit.

(e) The address of invlpg is recorded in exit-qualification. Upon a vmread of EXIT_QUALIFICATION the value obtained is:

Notice that the value recorded is a non-canonical address ie; address[63:48] != address[47]. This is the only case i have encountered where a non-canonical address shows up on the exit-qualification.

The only explanation I can come up with for this behavior is that : INVLPG unlike other instructions does not fault in 64-bit mode with a non-canonical operand. According to the instruction spec, INVLPG morphs into a NOP for such cases.

When a vmexit handler for INVLPG is written, this case must be taken into consideration(ie; a non-canonical address might show up in the exit-qualification field).