Re: [RFC PATCH] iommu: Default to lazy DMA mode on ARM64

[Pranjal Shrivastava <praan@google.com>]

Hi Nafees,

On Thu, Apr 02, 2026 at 07:59:13PM +0000, Nafees Ahmed Abdul wrote:
> ARM64 currently falls through to IOMMU_DEFAULT_DMA_STRICT, while
> X86 defaults to IOMMU_DEFAULT_DMA_LAZY. On ARM64 bare-metal
> systems with the ARM SMMU, strict mode causes synchronous TLBI
> + CMD_SYNC on every DMA unmap, resulting in significant
> throughput degradation for network-intensive workloads.
> 
> Benchmarked on an ARM64 bare-metal system (AWS m8g.metal-24xl)
> running Debian 13 with kernel 6.12.74, using iperf3:
> 
>   STRICT (default): 14.9 Gbps
>   LAZY:             39.8 Gbps
> 
> This is a 2.67x throughput improvement simply by switching the
> IOMMU default domain mode.
> 
> Distributions that do not explicitly override this Kconfig
> choice (e.g., Debian, SLES) silently get STRICT on ARM64,
> causing this regression on bare-metal systems. Changing the
> upstream default avoids the need for each distribution to
> independently carry this override.
> 

Thanks for the patch and the benchmarks.

However, I'm not sure why should we change the compile-time default for
all ARM64 systems? Currently, users can already achieve this behavior by
using the `iommu.strict=0` boot parameter.

Since IOMMU_DEFAULT_DMA_STRICT provides a higher security guarantee 
(preventing sub-page aliasing and potential "use-after-unmap" attacks),
keeping it as the default and allowing users to opt-in via the kernel cmd
line seems like the safer path, in my opinion. 

Additionally, distributions like Debian can also set this via their 
GRUB configurations for performance.

> Add ARM64 to the LAZY default to align with X86 behavior.
> 
> Signed-off-by: Nafees Ahmed Abdul <nafeabd@amazon.com>
> ---
>  drivers/iommu/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index f86262b11..2822aba75 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -96,7 +96,7 @@ config IOMMU_DEBUGFS
>  choice
>  	prompt "IOMMU default domain type"
>  	depends on IOMMU_API
> -	default IOMMU_DEFAULT_DMA_LAZY if X86 || S390
> +	default IOMMU_DEFAULT_DMA_LAZY if X86 || S390 || ARM64
>  	default IOMMU_DEFAULT_DMA_STRICT
>  	help
>  	  Choose the type of IOMMU domain used to manage DMA API usage by
 
Thanks,
Praan