Re: [PATCH] iommu/amd: Default to passthrough mode for improved performance

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

On 2026-03-26 9:38 am, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> On x86 platforms, AMD IOMMU is typically enabled by default. When the
> kernel is compiled with CONFIG_IOMMU_DEFAULT_DMA_LAZY, the IOMMU
> operates in translated mode with deferred TLB flushing. While this
> provides a security layer, it introduces measurable performance
> overhead compared to Intel systems where the IOMMU often defaults
> to a disabled state.
> 
> To optimize out-of-the-box performance for AMD users, shift the
> default to passthrough mode when the following conditions are met:
> 1. No explicit IOMMU mode was requested via the command line.
> 2. The kernel was configured to use 'lazy' DMA remapping by default.
> 3. Memory encryption (SME/SEV) is not active, as these features
>     require translation for security.
> 
> This change allows standard DMA operations to bypass remapping
> overhead while maintaining the ability for users to explicitly
> enable translation if required.
> 
> To support this, export iommu_dma_is_user_configured() from the
> IOMMU core to allow vendor drivers to check if the DMA API
> configuration was overridden by the user.

Frankly, no. CONFIG_IOMMU_DEFAULT_PASSTHROUGH already exists for users 
who want that behaviour. IF you want an equivalent of 
CONFIG_INTEL_IOMMU_DEFAULT_ON which prevents the IOMMU being used at all 
then implement that (however I imagine a lot of VFIO users would be 
unhappy about changing the default of that at this point). You can't 
just completely break CONFIG_IOMMU_DEFAULT_DMA_LAZY for all the users 
who do want its particular behaviour.

Note that "lazy" mode does still represent nearly all of the 
security/memory safety functionality offered by the IOMMU, so it does 
have significant value - strict mode only adds protection for 
use-after-free of memory which _was_ already a legitimate DMA buffer for 
the given device at one point. A better title for this patch would be 
"Silently make AMD systems less secure unless users go out of their way 
to add command-line arguments to work around this change"...

There may well also still be some performance difference between the 
IOMMU being enabled in passthrough, and being truly disabled - I seem to 
recall the Intel GPU folks saying that was significant enough to care 
about at least on some older Intel systems.

Thansk,
Robin.

> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>   drivers/iommu/amd/init.c | 9 +++++++++
>   drivers/iommu/iommu.c    | 6 ++++++
>   include/linux/iommu.h    | 1 +
>   3 files changed, 16 insertions(+)
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index f3fd7f3..e89a5ce 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -3619,6 +3619,15 @@ void __init amd_iommu_detect(void)
>   	amd_iommu_detected = true;
>   	iommu_detected = 1;
>   	x86_init.iommu.iommu_init = amd_iommu_init;
> +
> +	if (!iommu_dma_is_user_configured()) {
> +		if (!cc_platform_has(CC_ATTR_MEM_ENCRYPT) &&
> +				IS_ENABLED(CONFIG_IOMMU_DEFAULT_DMA_LAZY)) {
> +			pr_info("Defaulting to Passthrough mode for performance\n");
> +			iommu_set_default_passthrough(false);
> +		}
> +	}
> +
>   	return;
>   
>   disable_snp:
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 50718ab..a950dbb 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -4091,3 +4091,9 @@ int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
>   	return ret;
>   }
>   #endif /* CONFIG_IRQ_MSI_IOMMU */
> +
> +bool iommu_dma_is_user_configured(void)
> +{
> +	return !!(iommu_cmd_line & IOMMU_CMD_LINE_DMA_API);
> +}
> +EXPORT_SYMBOL_GPL(iommu_dma_is_user_configured);
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 54b8b48..c3ff8a9 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -967,6 +967,7 @@ int iommu_set_pgtable_quirks(struct iommu_domain *domain,
>   		unsigned long quirks);
>   
>   void iommu_set_dma_strict(void);
> +bool iommu_dma_is_user_configured(void);
>   
>   extern int report_iommu_fault(struct iommu_domain *domain, struct device *dev,
>   			      unsigned long iova, int flags);