iommu: Add fast hook for getting DMA domains

While iommu_get_domain_for_dev() is the robust way for arbitrary IOMMU
API callers to retrieve the domain pointer, for DMA ops domains it
doesn't scale well for large systems and multi-queue devices, since the
momentary refcount adjustment will lead to exclusive cacheline contention
when multiple CPUs are operating in parallel on different mappings for
the same device.

In the case of DMA ops domains, however, this refcounting is actually
unnecessary, since they already imply that the group exists and is
managed by platform code and IOMMU internals (by virtue of
iommu_group_get_for_dev()) such that a reference will already be held
for the lifetime of the device. Thus we can avoid the bottleneck by
providing a fast lookup specifically for the DMA code to retrieve the
default domain it already knows it has set up - a simple read-only
dereference plays much nicer with cache-coherency protocols.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

1 files changed, 9 insertions, 0 deletions

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 8c15c5980299..9d70344204fe 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1416,6 +1416,15 @@ struct iommu_domain *iommu_get_domain_for_dev(struct device *dev)
 EXPORT_SYMBOL_GPL(iommu_get_domain_for_dev);
 
 /*
+ * For IOMMU_DOMAIN_DMA implementations which already provide their own
+ * guarantees that the group and its default domain are valid and correct.
+ */
+struct iommu_domain *iommu_get_dma_domain(struct device *dev)
+{
+	return dev->iommu_group->default_domain;
+}
+
+/*
  * IOMMU groups are really the natrual working unit of the IOMMU, but
  * the IOMMU API works on domains and devices.  Bridge that gap by
  * iterating over the devices in a group.  Ideally we'd have a single