[PATCH v1 0/4] vfio/iommu: Flag to allow userspace to set DMA buffers system cacheable

[Manoj Vishwanathan <manojvishy@google.com>]

Hi maintainers,

This RFC patch introduces the ability for userspace to control whether
device (DMA) buffers are marked as cacheable, enabling them to utilize
the system-level cache.

The specific changes made in this patch are:

* Introduce a new flag in `include/linux/iommu.h`: 
    * `IOMMU_SYS_CACHE` -  Indicates if the associated page should be cached in the system's cache hierarchy.
* Add `VFIO_DMA_MAP_FLAG_SYS_CACHE` to `include/uapi/linux/vfio.h`:
    * Allows userspace to set the cacheable attribute to PTE when mapping DMA regions using the VFIO interface.
* Update `vfio_iommu_type1.c`:
    * Handle the `VFIO_DMA_MAP_FLAG_SYS_CACHE` flag during DMA mapping operations.
    * Set the `IOMMU_SYS_CACHE` flag in the IOMMU page table entry if the `VFIO_DMA_MAP_FLAG_SYS_CACHE` is set.

* arm/smmu/io-pgtable-arm: Set the MAIR for SYS_CACHE

The reasoning behind these changes is to provide userspace with finer-grained control over memory access patterns for devices,
potentially improving performance in scenarios where caching is beneficial. We saw in some of the use cases where the buffers were
for transient data ( in and out without processing).

I have tested this patch on certain arm64 machines and observed the following:

* There is 14-21% improvement in jitter measurements, where the buffer on System Level Cache vs DDR buffers
* There was not much of an improvement in latency in the diration of the tests that I have tried.

I am open to feedback and suggestions for further improvements. Please let me know if you have any questions or concerns.
Also, I am working on adding a check in the VFIO layer to ensure that if there is no architecture supported implementation for
sys cache, if should not apply them.

Thanks,
Manoj Vishwanathan

Manoj Vishwanathan (4):
  iommu: Add IOMMU_SYS_CACHE flag for system cache control
  iommu/io-pgtable-arm: Force outer cache for page-level MAIR via user
    flag
  vfio: Add VFIO_DMA_MAP_FLAG_SYS_CACHE to control device access to
    system cache
  vfio/type1: Add support for VFIO_DMA_MAP_FLAG_SYS_CACHE

 drivers/iommu/io-pgtable-arm.c  | 3 +++
 drivers/vfio/vfio_iommu_type1.c | 5 +++--
 include/linux/iommu.h           | 6 ++++++
 include/uapi/linux/vfio.h       | 1 +
 4 files changed, 13 insertions(+), 2 deletions(-)

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2.46.0.295.g3b9ea8a38a-goog