From 6af588fed39178c8e118fcf9cb6664e58a1fbe88 Mon Sep 17 00:00:00 2001 From: Robin Murphy Date: Wed, 12 Sep 2018 16:24:12 +0100 Subject: [PATCH] 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 Tested-by: Will Deacon Signed-off-by: Joerg Roedel --- drivers/iommu/iommu.c | 9 +++++++++ include/linux/iommu.h | 1 + 2 files changed, 10 insertions(+) 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 @@ -1415,6 +1415,15 @@ 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 diff --git a/include/linux/iommu.h b/include/linux/iommu.h index 87994c265bf5..c783648d4060 100644 --- a/include/linux/iommu.h +++ b/include/linux/iommu.h @@ -293,6 +293,7 @@ extern int iommu_attach_device(struct iommu_domain *domain, extern void iommu_detach_device(struct iommu_domain *domain, struct device *dev); extern struct iommu_domain *iommu_get_domain_for_dev(struct device *dev); +extern struct iommu_domain *iommu_get_dma_domain(struct device *dev); extern int iommu_map(struct iommu_domain *domain, unsigned long iova, phys_addr_t paddr, size_t size, int prot); extern size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova,