netdev - Re: [PATCH 16/18] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH

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Message-ID: <20210330131149.GP5908@willie-the-truck>
Date:   Tue, 30 Mar 2021 14:11:49 +0100
From:   Will Deacon <will@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Joerg Roedel <joro@...tes.org>, Li Yang <leoyang.li@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 16/18] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE

On Tue, Mar 16, 2021 at 04:38:22PM +0100, Christoph Hellwig wrote:
> From: Robin Murphy <robin.murphy@....com>
> 
> Instead make the global iommu_dma_strict paramete in iommu.c canonical by
> exporting helpers to get and set it and use those directly in the drivers.
> 
> This make sure that the iommu.strict parameter also works for the AMD and
> Intel IOMMU drivers on x86.  As those default to lazy flushing a new
> IOMMU_CMD_LINE_STRICT is used to turn the value into a tristate to
> represent the default if not overriden by an explicit parameter.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>.
> [ported on top of the other iommu_attr changes and added a few small
>  missing bits]
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  drivers/iommu/amd/iommu.c                   | 23 +-------
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 50 +---------------
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  1 -
>  drivers/iommu/arm/arm-smmu/arm-smmu.c       | 27 +--------
>  drivers/iommu/dma-iommu.c                   |  9 +--
>  drivers/iommu/intel/iommu.c                 | 64 ++++-----------------
>  drivers/iommu/iommu.c                       | 27 ++++++---
>  include/linux/iommu.h                       |  4 +-
>  8 files changed, 40 insertions(+), 165 deletions(-)

I really like this cleanup, but I can't help wonder if it's going in the
wrong direction. With SoCs often having multiple IOMMU instances and a
distinction between "trusted" and "untrusted" devices, then having the
flush-queue enabled on a per-IOMMU or per-domain basis doesn't sound
unreasonable to me, but this change makes it a global property.

For example, see the recent patch from Lu Baolu:

https://lore.kernel.org/r/20210225061454.2864009-1-baolu.lu@linux.intel.com

Will