KVM: vmx: rewrite the comment in vmx_get_mt_mask

	u8 cache;

	u64 ipat = 0;

	/* For VT-d and EPT combination

	 * 1. MMIO: always map as UC

	 * 2. EPT with VT-d:

	 *   a. VT-d without snooping control feature: can't guarantee the

	 *	result, try to trust guest.

	 *   b. VT-d with snooping control feature: snooping control feature of

	 *	VT-d engine can guarantee the cache correctness. Just set it

	 *	to WB to keep consistent with host. So the same as item 3.

	 * 3. EPT without VT-d: always map as WB and set IPAT=1 to keep

	 *    consistent with host MTRR

	/* We wanted to honor guest CD/MTRR/PAT, but doing so could result in

	 * memory aliases with conflicting memory types and sometimes MCEs.

	 * We have to be careful as to what are honored and when.

	 *

	 * For MMIO, guest CD/MTRR are ignored.  The EPT memory type is set to

	 * UC.  The effective memory type is UC or WC depending on guest PAT.

	 * This was historically the source of MCEs and we want to be

	 * conservative.

	 *

	 * When there is no need to deal with noncoherent DMA (e.g., no VT-d

	 * or VT-d has snoop control), guest CD/MTRR/PAT are all ignored.  The

	 * EPT memory type is set to WB.  The effective memory type is forced

	 * WB.

	 *

	 * Otherwise, we trust guest.  Guest CD/MTRR/PAT are all honored.  The

	 * EPT memory type is used to emulate guest CD/MTRR.

	 */

	if (is_mmio) {

		cache = MTRR_TYPE_UNCACHABLE;

		goto exit;