[PATCH 0/4] Add dynamic iommu backed bounce buffers

[David Stevens]

Add support for per-domain dynamic pools of iommu bounce buffers to the 
dma-iommu API. This allows iommu mappings to be reused while still
maintaining strict iommu protection. Allocating buffers dynamically
instead of using swiotlb carveouts makes per-domain pools more amenable
on systems with large numbers of devices or where devices are unknown.

When enabled, all non-direct streaming mappings below a configurable
size will go through bounce buffers. Note that this means drivers which
don't properly use the DMA API (e.g. i915) cannot use an iommu when this
feature is enabled. However, all drivers which work with swiotlb=force
should work.

Bounce buffers serve as an optimization in situations where interactions
with the iommu are very costly. For example, virtio-iommu operations in
a guest on a linux host require a vmexit, involvement the VMM, and a
VFIO syscall. For relatively small DMA operations, memcpy can be
significantly faster.

As a performance comparison, on a device with an i5-10210U, I ran fio
with a VFIO passthrough NVMe drive with '--direct=1 --rw=read
--ioengine=libaio --iodepth=64' and block sizes 4k, 16k, 64k, and
128k. Test throughput increased by 2.8x, 4.7x, 3.6x, and 3.6x. Time
spent in iommu_dma_unmap_(page|sg) per GB processed decreased by 97%,
94%, 90%, and 87%. Time spent in iommu_dma_map_(page|sg) decreased
by >99%, as bounce buffers don't require syncing here in the read case.
Running with multiple jobs doesn't serve as a useful performance
comparison because virtio-iommu and vfio_iommu_type1 both have big
locks that significantly limit mulithreaded DMA performance.

This patch set is based on v5.13-rc7 plus the patches at [1].

David Stevens (4):
  dma-iommu: add kalloc gfp flag to alloc helper
  dma-iommu: replace device arguments
  dma-iommu: expose a few helper functions to module
  dma-iommu: Add iommu bounce buffers to dma-iommu api

 drivers/iommu/Kconfig          |  10 +
 drivers/iommu/Makefile         |   1 +
 drivers/iommu/dma-iommu.c      | 119 ++++--
 drivers/iommu/io-buffer-pool.c | 656 +++++++++++++++++++++++++++++++++
 drivers/iommu/io-buffer-pool.h |  91 +++++
 include/linux/dma-iommu.h      |  12 +
 6 files changed, 861 insertions(+), 28 deletions(-)
 create mode 100644 drivers/iommu/io-buffer-pool.c
 create mode 100644 drivers/iommu/io-buffer-pool.h

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2.32.0.93.g670b81a890-goog