iommu/vt-d: Clean up pasid_enabled() and ecs_enabled() dependencies

When booted with intel_iommu=ecs_off we were still allocating the PASID
tables even though we couldn't actually use them. We really want to make
the pasid_enabled() macro depend on ecs_enabled().

Which is unfortunate, because currently they're the other way round to
cope with the Broadwell/Skylake problems with ECS.

Instead of having ecs_enabled() depend on pasid_enabled(), which was never
something that made me happy anyway, make it depend in the normal case
on the "broken PASID" bit 28 *not* being set.

Then pasid_enabled() can depend on ecs_enabled() as it should. And we also
don't need to mess with it if we ever see an implementation that has some
features requiring ECS (like PRI) but which *doesn't* have PASID support.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/iommu/intel-iommu.c

@@ -507,14 +507,30 @@ static int iommu_identity_mapping;
 #define IDENTMAP_GFX		2
 #define IDENTMAP_AZALIA		4
 
-/* We only actually use ECS when PASID support (on the new bit 40)
- * is also advertised. Some early implementations — the ones with
- * PASID support on bit 28 — have issues even when we *only* use
- * extended root/context tables. */
-#define pasid_enabled(iommu) (ecap_pasid(iommu->ecap) || \
-			      (intel_iommu_pasid28 && ecap_broken_pasid(iommu->ecap)))
+/* Broadwell and Skylake have broken ECS support — normal so-called "second
+ * level" translation of DMA requests-without-PASID doesn't actually happen
+ * unless you also set the NESTE bit in an extended context-entry. Which of
+ * course means that SVM doesn't work because it's trying to do nested
+ * translation of the physical addresses it finds in the process page tables,
+ * through the IOVA->phys mapping found in the "second level" page tables.
+ *
+ * The VT-d specification was retroactively changed to change the definition
+ * of the capability bits and pretend that Broadwell/Skylake never happened...
+ * but unfortunately the wrong bit was changed. It's ECS which is broken, but
+ * for some reason it was the PASID capability bit which was redefined (from
+ * bit 28 on BDW/SKL to bit 40 in future).
+ *
+ * So our test for ECS needs to eschew those implementations which set the old
+ * PASID capabiity bit 28, since those are the ones on which ECS is broken.
+ * Unless we are working around the 'pasid28' limitations, that is, by putting
+ * the device into passthrough mode for normal DMA and thus masking the bug.
+ */
 #define ecs_enabled(iommu) (intel_iommu_ecs && ecap_ecs(iommu->ecap) && \
-			    pasid_enabled(iommu))
+			    (intel_iommu_pasid28 || !ecap_broken_pasid(iommu->ecap)))
+/* PASID support is thus enabled if ECS is enabled and *either* of the old
+ * or new capability bits are set. */
+#define pasid_enabled(iommu) (ecs_enabled(iommu) &&			\
+			      (ecap_pasid(iommu->ecap) || ecap_broken_pasid(iommu->ecap)))
 
 int intel_iommu_gfx_mapped;
 EXPORT_SYMBOL_GPL(intel_iommu_gfx_mapped);