linux-kernel - Re: [PATCH 00/13] Introduce IOMMU-API TLB Flushing Interface

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Date:   Thu, 17 Aug 2017 08:35:20 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Subject: Re: [PATCH 00/13] Introduce IOMMU-API TLB Flushing Interface

On Thu, 17 Aug 2017 14:56:23 +0200
Joerg Roedel <joro@...tes.org> wrote:

> Hi,
> 
> here is a patch-set to introduce an explicit interface to
> the IOMMU-API to flush IOMMU and device IO/TLBs. Currently
> the iommu_map(), iommu_map_sg(), and iommu_unmap() functions
> have to make sure all IO/TLBs in the system are synchronized
> with the page-table updates they made.
> 
> This is very inefficient in some scenarios, for example when
> a large address space is unmapped and an IO/TLB flush has to
> be done in every call of iommu_unmap(). Or in a scenario
> where it makes sense to queue up some changes to the
> page-tables and flush them together.
> 
> To optimize these scenarios, the need to synchronize with
> the IOMMU and device TLBs has been removed from the
> map/unmap functions of the IOMMU-API and an interface to
> explicitly do the flushes has been introduced.
> 
> To make the conversion of existing users of the IOMMU-API
> easier, new functions - iommu_map_sync(), iommu_map_sg_sync(),
> and iommu_unmap_sync() - have been introduced. These
> functions guarantee that the IO/TLBs are synchronized with
> any page-table update when they return. The optimizations
> possible with the new interface are subject to separate
> patch-sets.

Hi Joerg,

Wouldn't it be much more friendly to downstreams and out-of-tree
drivers to introduce new functions for the async semantics?  ie.
iommu_map_async(), etc.  The API also seems a little cleaner that
iommu_map() stands alone, it's synchronous, iommu_map_async() is
explicitly asynchronous and a _flush() call is needed to finalize it.
What do you see as the advantage to the approach here?  Thanks,

Alex


> Patch 1 just renames a few functions in the AMD-Vi driver
> that would otherwise collide with the new TLB-flush
> functions from the IOMMU-API.
> 
> Patch 2 introduces the new IO/TLB Flush-Interface.
> 
> Patch 3-13 convert existing users of the IOMMU-API to use
> the *_sync functions for now.
> 
> Please review.
> 
> Thanks,
> 
> 	Joerg
> 
> Joerg Roedel (13):
>   iommu/amd: Rename a few flush functions
>   iommu: Introduce Interface for IOMMU TLB Flushing
>   vfio/type1: Use sychronized interface of the IOMMU-API
>   iommu/dma: Use sychronized interface of the IOMMU-API
>   arm: dma-mapping: Use sychronized interface of the IOMMU-API
>   drm/etnaviv: Use sychronized interface of the IOMMU-API
>   drm/msm: Use sychronized interface of the IOMMU-API
>   drm/nouveau/imem/gk20a: Use sychronized interface of the IOMMU-API
>   drm/rockchip: Use sychronized interface of the IOMMU-API
>   drm/tegra: Use sychronized interface of the IOMMU-API
>   gpu: host1x: Use sychronized interface of the IOMMU-API
>   IB/usnic: Use sychronized interface of the IOMMU-API
>   remoteproc: Use sychronized interface of the IOMMU-API
> 
>  arch/arm/mm/dma-mapping.c                          | 21 +++---
>  drivers/gpu/drm/etnaviv/etnaviv_mmu.c              | 10 +--
>  drivers/gpu/drm/msm/msm_iommu.c                    |  5 +-
>  .../gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c    | 12 ++--
>  drivers/gpu/drm/rockchip/rockchip_drm_gem.c        |  6 +-
>  drivers/gpu/drm/tegra/drm.c                        |  6 +-
>  drivers/gpu/drm/tegra/gem.c                        |  6 +-
>  drivers/gpu/host1x/cdma.c                          |  6 +-
>  drivers/gpu/host1x/job.c                           |  6 +-
>  drivers/infiniband/hw/usnic/usnic_uiom.c           | 10 +--
>  drivers/iommu/amd_iommu.c                          | 16 ++---
>  drivers/iommu/dma-iommu.c                          |  8 +--
>  drivers/iommu/iommu.c                              | 26 +++++++
>  drivers/remoteproc/remoteproc_core.c               | 10 +--
>  drivers/vfio/vfio_iommu_type1.c                    | 38 +++++-----
>  include/linux/iommu.h                              | 80 +++++++++++++++++++++-
>  16 files changed, 189 insertions(+), 77 deletions(-)
>