IO operations and MOV to/from CR3

Oct. 27, 2012

      Hi List, 

I prepared some notes for CR3 involvement in different IO
operations, so here are the notes

-	memory-mapped IO & DMA IO:	
	-	Each such operation involves MOV to/from CR3 :
	-	MOV from CR3 causes "Unconditional VM exit"
         -	MOV to CR3 causes "Conditional VM exit" :
	        -	The MOV to CR3 instruction causes a VM exit unless the value
of its
			source operand is equal to one of the CR3-target values specified in 
			the VMCS (controlled by Hypervisor). This feature allows a guest
kernel to 
			change the CR3 value without causing a world switch to the
hypervisor,
			provided that the value written into the CR3 register was previously 
			specified by the hypervisor in the CR3-Target registers. An
execution 
			of MOV to CR3 in VMX non-root operation does not cause a VM exit if 
			its source operand matches one of these previsously stored values. 
			If the CR3-target count is n, only the first n (0 .. (n-1))
CR3-target 
			values do not cause VM-Exit. nth value causes VM-Exit. if n=0 , 
                         MOV to CR3 always causes VM-Exit.

Please comment if I understood something incorrectly here.

Regards,
K Arun Kumar