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-rw-r--r--drivers/iommu/intel-iommu.c30
1 files changed, 23 insertions, 7 deletions
diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 7a1c15de418a..2973c09c9677 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/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);