Re: [RFC: DMA_PMD 01/22] iommu/dma: introduce CONFIG_DMA_PMD and metadata table

[Randy Dunlap]

On 10/3/26 2:22 PM, Luigi Rizzo wrote:
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index 6e07bd69467a3..1bb347fd9da4a 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -158,6 +158,38 @@ config IOMMU_DMA
>  	select NEED_SG_DMA_LENGTH
>  	select NEED_SG_DMA_FLAGS if SWIOTLB
>  
> +# PMD_SIZE IOMMU backing for DMA-coherent devices
> +config DMA_PMD
> +	bool "PMD_SIZE IOMMU backing for DMA-coherent devices"
> +	depends on IOMMU_DMA
> +	depends on X86_64 || (ARM64 && ARM64_4K_PAGES)
> +	depends on !PREEMPT_RT
> +	default y
> +	help
> +	  Hand out DMA buffers carved out of PMD_SIZE physically contiguous
> +	  blocks that the IOMMU maps with a single leaf PTE. Mapping,
> +	  unmapping and IOTLB flushing are done lazily to reduce CPU overhead.
> +	  The large mappings greatly reduce IOTLB usage. As in strict iommu
> +	  mode, pages are returned to the buddy allocator only when any existing
> +	  mappings have been removed and IOTLB flushed.
> +	  Costs 128KB of metadata per GB of DMA_PMD buffers, allocated on demand.

Paragraph 1 of the patch description says 64KB per 1GB, not 128KB.
Or am I mixing things up?

-- 
~Randy