[PATCH v11 4/6] iommu/s390: Force ISM devices to use IOMMU_DOMAIN_DMA

[Niklas Schnelle <schnelle@linux.ibm.com>]

ISM devices are virtual PCI devices used for cross-LPAR communication.
Unlike real PCI devices ISM devices do not use the hardware IOMMU but
inspects IOMMU translation tables directly on IOTLB flush (s390 RPCIT
instruction).

While ISM devices keep their DMA allocations static and only very rarely
DMA unmap at all, For each IOTLB flush that occurs after unmap the ISM
devices will inspect the area of the IOVA space indicated by the flush.
This means that for the global IOTLB flushes used by the flush queue
mechanism the entire IOVA space would be inspected. In principle this
would be fine, albeit potentially unnecessarily slow, it turns out
however that ISM devices are sensitive to seeing IOVA addresses that are
currently in use in the IOVA range being flushed. Seeing such in-use
IOVA addresses will cause the ISM device to enter an error state and
become unusable.

Fix this by forcing IOMMU_DOMAIN_DMA to be used for ISM devices. This
makes sure IOTLB flushes only cover IOVAs that have been unmapped and
also restricts the range of the IOTLB flush potentially reducing latency
spikes.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
 drivers/iommu/s390-iommu.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index f6d6c60e5634..020cc538e4c4 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c
@@ -710,6 +710,15 @@ struct zpci_iommu_ctrs *zpci_get_iommu_ctrs(struct zpci_dev *zdev)
 	return &zdev->s390_domain->ctrs;
 }
 
+static int s390_iommu_def_domain_type(struct device *dev)
+{
+	struct zpci_dev *zdev = to_zpci_dev(dev);
+
+	if (zdev->pft == PCI_FUNC_TYPE_ISM)
+		return IOMMU_DOMAIN_DMA;
+	return 0;
+}
+
 int zpci_init_iommu(struct zpci_dev *zdev)
 {
 	u64 aperture_size;
@@ -789,6 +798,7 @@ static const struct iommu_ops s390_iommu_ops = {
 	.probe_device = s390_iommu_probe_device,
 	.probe_finalize = s390_iommu_probe_finalize,
 	.release_device = s390_iommu_release_device,
+	.def_domain_type = s390_iommu_def_domain_type,
 	.device_group = generic_device_group,
 	.pgsize_bitmap = SZ_4K,
 	.get_resv_regions = s390_iommu_get_resv_regions,

-- 
2.39.2