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Date:   Fri, 30 Jul 2021 11:28:23 +0200
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     joro@8bytes.org, will@kernel.org, linux-kernel@vger.kernel.org,
        dianders@chromium.org, iommu@lists.linux-foundation.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 19/24] iommu: Expose DMA domain strictness via sysfs
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References: <cover.1627468308.git.robin.murphy@arm.com>
 <dd26592103c7613ef9fdff703d0d2ed2df8305c2.1627468310.git.robin.murphy@arm.com>
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On Wed, Jul 28, 2021 at 04:58:40PM +0100, Robin Murphy wrote:
> The sysfs interface for default domain types exists primarily so users
> can choose the performance/security tradeoff relevant to their own
> workload. As such, the choice between the policies for DMA domains fits
> perfectly as an additional point on that scale - downgrading a
> particular device from a strict default to non-strict may be enough to
> let it reach the desired level of performance, while still retaining
> more peace of mind than with a wide-open identity domain. Now that we've
> abstracted non-strict mode as a distinct type of DMA domain, allow it to
> be chosen through the user interface as well.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  Documentation/ABI/testing/sysfs-kernel-iommu_groups | 2 ++
>  drivers/iommu/iommu.c                               | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-kernel-iommu_groups b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> index eae2f1c1e11e..43ba764ba5b7 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> +++ b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> @@ -42,6 +42,8 @@ Description:	/sys/kernel/iommu_groups/<grp_id>/type shows the type of default
>  		========  ======================================================
>  		DMA       All the DMA transactions from the device in this group
>  		          are translated by the iommu.
> +		DMA-FQ    As above, but using batched invalidation to lazily
> +		          remove translations after use.

It might be useful to desribe the security/performance tradeoff here as
well. I guess a normal user is more likely to look at the doc for this
sysfs knob than the kernel config or parameters.

Thanks,
Jean

>  		identity  All the DMA transactions from the device in this group
>  		          are not translated by the iommu.
>  		auto      Change to the type the device was booted with.
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index eecb5657de69..5a08e0806cbb 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -3265,6 +3265,8 @@ static ssize_t iommu_group_store_type(struct iommu_group *group,
>  		req_type = IOMMU_DOMAIN_IDENTITY;
>  	else if (sysfs_streq(buf, "DMA"))
>  		req_type = IOMMU_DOMAIN_DMA;
> +	else if (sysfs_streq(buf, "DMA-FQ"))
> +		req_type = IOMMU_DOMAIN_DMA_FQ;
>  	else if (sysfs_streq(buf, "auto"))
>  		req_type = 0;
>  	else
> -- 
> 2.25.1
> 
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