intel-iommu: ignore page table validation in pass through mode

We are seeing a bug when booting w/ iommu=pt with current upstream
(bisect blames 19943b0e30b05d42e494ae6fef78156ebc8c637e "intel-iommu:
Unify hardware and software passthrough support).

The issue is specific to this loop during identity map initialization
of each device:

domain_context_mapping_one(si_domain, ..., CONTEXT_TT_PASS_THROUGH)
...
		/* Skip top levels of page tables for
		* iommu which has less agaw than default.
		*/
		for (agaw = domain->agaw; agaw != iommu->agaw; agaw--) {
			pgd = phys_to_virt(dma_pte_addr(pgd));
			if (!dma_pte_present(pgd)) {      <------ failing here
				spin_unlock_irqrestore(&iommu->lock, flags);
			return -ENOMEM;
		}

This box has 2 iommu's in it.  The catchall iommu has MGAW == 48, and
SAGAW == 4.  The other iommu has MGAW == 39, SAGAW == 2.

The device that's failing the above pgd test is the only device connected
to the non-catchall iommu, which has a smaller address width than the
domain default.  This test is not necessary since the context is in PT
mode and the ASR is ignored.

Thanks to Don Dutile for discovering and debugging this one.

Cc: stable@kernel.org
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/pci/intel-iommu.c

@@ -1530,12 +1530,15 @@ static int domain_context_mapping_one(struct dmar_domain *domain, int segment,
 
 		/* Skip top levels of page tables for
 		 * iommu which has less agaw than default.
+		 * Unnecessary for PT mode.
 		 */
-		for (agaw = domain->agaw; agaw != iommu->agaw; agaw--) {
-			pgd = phys_to_virt(dma_pte_addr(pgd));
-			if (!dma_pte_present(pgd)) {
-				spin_unlock_irqrestore(&iommu->lock, flags);
-				return -ENOMEM;
+		if (translation != CONTEXT_TT_PASS_THROUGH) {
+			for (agaw = domain->agaw; agaw != iommu->agaw; agaw--) {
+				pgd = phys_to_virt(dma_pte_addr(pgd));
+				if (!dma_pte_present(pgd)) {
+					spin_unlock_irqrestore(&iommu->lock, flags);
+					return -ENOMEM;
+				}
 			}
 		}
 	}