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

[Robin Murphy]

On 02/04/2026 8:59 pm, 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.

It is not a "regression", it has always been this way since the 
beginning of IOMMU support on arm64. For many years, we didn't even have 
such a thing as lazy mode.

> Changing the
> upstream default avoids the need for each distribution to
> independently carry this override.

...while equally *creating* that need for all the distros/users who do 
value security/robustness above performance. Who's to say what matters 
most? Besides, defconfig is never meant to be a distro config; distros 
*should* maintain their own configs, and if they're not delivering the 
options that the majority of their users want, that's between the 
distros and their users.

The numbers game goes both ways too - the sheer quantity of arm64 
systems where strict vs. lazy makes no noticeable performance 
difference, but does offer that small robustness benefit (i.e. 
embedded/mobile) is many orders of magnitude more the number of arm64 
systems capable of 50GbE. Even your own data are suggesting this is 
actually a pretty niche case, if even 10GbE systems would still have 
plenty of headroom to keep up in strict mode - if anything that's 
actually pretty impressive!

> Add ARM64 to the LAZY default to align with X86 behavior.

But the other side of that is that the x86 (and S390) behaviour is a 
20-year-old legacy which arguably only looks more and more anachronistic 
in today's post-Spectre/etc. security-conscious world. Wouldn't an even 
better alignment argument be to start cleaning up such legacy, rather 
than spread it further onto more modern architectures which never even 
had it?

Thanks,
Robin.

> Signed-off-by: Nafees Ahmed Abdul <nafeabd@xxxxxxxxxx>
> ---
>   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