Re: [PATCH v12 2/5] iommu: Enhance IOMMU default DMA mode build options

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-11 13:20, John Garry wrote:
> From: Zhen Lei <thunder.leizhen@huawei.com>
> 
> First, add build options IOMMU_DEFAULT_{LAZY|STRICT}, so that we have the
> opportunity to set {lazy|strict} mode as default at build time. Then put
> the two config options in an choice, as they are mutually exclusive.
> 
> [jpg: Make choice between strict and lazy only (and not passthrough)]
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
>   drivers/iommu/Kconfig | 38 ++++++++++++++++++++++++++++++++++++++
>   drivers/iommu/iommu.c |  3 ++-
>   2 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index 1f111b399bca..2a71347611d4 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -90,6 +90,44 @@ config IOMMU_DEFAULT_PASSTHROUGH
>   
>   	  If unsure, say N here.
>   
> +choice
> +	prompt "IOMMU default DMA mode"
> +	depends on IOMMU_DMA
> +
> +	default IOMMU_DEFAULT_STRICT
> +	help
> +	  This option allows an IOMMU DMA mode to be chosen at build time, to
> +	  override the default DMA mode of each ARCH, removing the need to
> +	  pass in kernel parameters through command line. It is still possible
> +	  to provide ARCH-specific or common boot options to override this
> +	  option.
> +
> +	  If unsure, keep the default.
> +
> +config IOMMU_DEFAULT_STRICT
> +	bool "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.
> +
> +config IOMMU_DEFAULT_LAZY
> +	bool "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.
> +
> +	  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 mapping finally being torn down in the IOMMU, where the
> +	  device can still access the system memory. However this mode may
> +	  provide better performance in high throughput scenarios, and is still
> +	  considerably more secure than passthrough mode or no IOMMU.
> +
> +endchoice
> +
>   config OF_IOMMU
>   	def_bool y
>   	depends on OF && IOMMU_API
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index cf58949cc2f3..ccbd5d4c1a50 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -29,7 +29,8 @@ 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 = true;
> +static bool iommu_dma_strict __read_mostly =
> +			IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);

Nit: I reckon this is a fine place to wield the 100-character 
relaxation, but it's not my call in the end.

With the wording tweaks already mentioned,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

>   static u32 iommu_cmd_line __read_mostly;
>   
>   struct iommu_group {
>