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Date:   Fri, 30 Jul 2021 11:29:38 +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 20/24] iommu: Merge strictness and domain type configs
Message-ID: <YQPGgjT5nGdbTYDN@myrica>
References: <cover.1627468308.git.robin.murphy@arm.com>
 <992b2952f0b173411c7b6f221dce82e8e082c0b8.1627468310.git.robin.murphy@arm.com>
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On Wed, Jul 28, 2021 at 04:58:41PM +0100, Robin Murphy wrote:
> To parallel the sysfs behaviour, merge the new build-time option
> for DMA domain strictness into the default domain type choice.
> 
> Suggested-by: Joerg Roedel <joro@8bytes.org>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

kernel-parameters.txt still refers to the old config options

Otherwise
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

> ---
>  drivers/iommu/Kconfig | 80 +++++++++++++++++++++----------------------
>  drivers/iommu/iommu.c |  2 +-
>  2 files changed, 41 insertions(+), 41 deletions(-)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index c84da8205be7..6e06f876d75a 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -79,55 +79,55 @@ config IOMMU_DEBUGFS
>  	  debug/iommu directory, and then populate a subdirectory with
>  	  entries as required.
>  
> -config IOMMU_DEFAULT_PASSTHROUGH
> -	bool "IOMMU passthrough by default"
> -	depends on IOMMU_API
> -	help
> -	  Enable passthrough by default, removing the need to pass in
> -	  iommu.passthrough=on or iommu=pt through command line. If this
> -	  is enabled, you can still disable with iommu.passthrough=off
> -	  or iommu=nopt depending on the architecture.
> -
> -	  If unsure, say N here.
> -
>  choice
> -	prompt "IOMMU default DMA IOTLB invalidation mode"
> -	depends on IOMMU_DMA
> -
> -	default IOMMU_DEFAULT_LAZY if (AMD_IOMMU || INTEL_IOMMU)
> -	default IOMMU_DEFAULT_STRICT
> +	prompt "IOMMU default domain type"
> +	depends on IOMMU_API
> +	default IOMMU_DEFAULT_DMA_LAZY if AMD_IOMMU || INTEL_IOMMU
> +	default IOMMU_DEFAULT_DMA_STRICT
>  	help
> -	  This option allows an IOMMU DMA IOTLB invalidation mode to be
> -	  chosen at build time, to override the default mode of each ARCH,
> -	  removing the need to pass in kernel parameters through command line.
> -	  It is still possible to provide common boot params to override this
> -	  config.
> +	  Choose the type of IOMMU domain used to manage DMA API usage by
> +	  device drivers. The options here typically represent different
> +	  levels of tradeoff between robustness/security and performance,
> +	  depending on the IOMMU driver. Not all IOMMUs support all options.
> +	  This choice can be overridden at boot via the command line, and for
> +	  some devices also at runtime via sysfs.
>  
>  	  If unsure, keep the default.
>  
> -config IOMMU_DEFAULT_STRICT
> -	bool "strict"
> +config IOMMU_DEFAULT_DMA_STRICT
> +	bool "Translated - Strict"
>  	help
> -	  For every IOMMU DMA unmap operation, the flush operation of IOTLB and
> -	  the free operation of IOVA are guaranteed to be done in the unmap
> -	  function.
> +	  Trusted devices use translation to restrict their access to only
> +	  DMA-mapped pages, with strict TLB invalidation on unmap. Equivalent
> +	  to passing "iommu.passthrough=0 iommu.strict=1" on the command line.
>  
> -config IOMMU_DEFAULT_LAZY
> -	bool "lazy"
> +	  Untrusted devices always use this mode, with an additional layer of
> +	  bounce-buffering such that they cannot gain access to any unrelated
> +	  data within a mapped page.
> +
> +config IOMMU_DEFAULT_DMA_LAZY
> +	bool "Translated - Lazy"
>  	help
> -	  Support lazy mode, where for every IOMMU DMA unmap operation, the
> -	  flush operation of IOTLB and the free operation of IOVA are deferred.
> -	  They are only guaranteed to be done before the related IOVA will be
> -	  reused.
> +	  Trusted devices use translation to restrict their access to only
> +	  DMA-mapped pages, but with "lazy" batched TLB invalidation. This
> +	  mode allows higher performance with some IOMMUs due to reduced TLB
> +	  flushing, but at the cost of reduced isolation since devices may be
> +	  able to access memory for some time after it has been unmapped.
> +	  Equivalent to passing "iommu.passthrough=0 iommu.strict=0" on the
> +	  command line.
>  
> -	  The isolation provided in this mode is not as secure as STRICT mode,
> -	  such that a vulnerable time window may be created between the DMA
> -	  unmap and the mappings cached in the IOMMU IOTLB or device TLB
> -	  finally being invalidated, where the device could still access the
> -	  memory which has already been unmapped by the device driver.
> -	  However this mode may provide better performance in high throughput
> -	  scenarios, and is still considerably more secure than passthrough
> -	  mode or no IOMMU.
> +	  If this mode is not supported by the IOMMU driver, the effective
> +	  runtime default will fall back to IOMMU_DEFAULT_DMA_STRICT.
> +
> +config IOMMU_DEFAULT_PASSTHROUGH
> +	bool "Passthrough"
> +	help
> +	  Trusted devices are identity-mapped, giving them unrestricted access
> +	  to memory with minimal performance overhead. Equivalent to passing
> +	  "iommu.passthrough=1" (historically "iommu=pt") on the command line.
> +
> +	  If this mode is not supported by the IOMMU driver, the effective
> +	  runtime default will fall back to IOMMU_DEFAULT_DMA_STRICT.
>  
>  endchoice
>  
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 5a08e0806cbb..25c1adc1ec67 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -31,7 +31,7 @@ static struct kset *iommu_group_kset;
>  static DEFINE_IDA(iommu_group_ida);
>  
>  static unsigned int iommu_def_domain_type __read_mostly;
> -static bool iommu_dma_strict __read_mostly = IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);
> +static bool iommu_dma_strict __read_mostly = IS_ENABLED(CONFIG_IOMMU_DEFAULT_DMA_STRICT);
>  static u32 iommu_cmd_line __read_mostly;
>  
>  struct iommu_group {
> -- 
> 2.25.1
> 
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