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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 26 Aug 2024 07:16:37 +0000
Manoj Vishwanathan <manojvishy@google.com> wrote:

> 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.

We intend to eventually drop vfio type1 in favor of using IOMMUFD,
therefore all new type1 features need to first be available in IOMMUFD.
Once there we may consider use cases for providing the feature in the
legacy type1 interface and the IOMMUFD compatibility interface.  Thanks,

Alex

> * 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(-)
>