Re: [PATCH v2 20/24] iommu: Merge strictness and domain type configs

Jean-Philippe Brucker
2021-07-30T09:31:50Z

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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