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

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Message-ID: <CA+M8utOvUr3LrWhTNtcBhRAsOB-ST8kKWJN4u_0eNCcWLihH4w@mail.gmail.com>
Date: Mon, 26 Aug 2024 10:36:58 -0700
From: Manoj Vishwanathan <manojvishy@...gle.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>, 
	Joerg Roedel <joro@...tes.org>, linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org, 
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	David Dillow <dillow@...gle.com>, Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 0/4] vfio/iommu: Flag to allow userspace to set DMA
 buffers system cacheable

On Mon, Aug 26, 2024 at 10:04 AM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Mon, 26 Aug 2024 07:16:37 +0000
> Manoj Vishwanathan <manojvishy@...gle.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
Thank You, Alex! I will redirect this patch to iommufd in the next version.
> > * 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(-)
> >
>