Re: [RFC: DMA_PMD 01/22] iommu/dma: introduce CONFIG_DMA_PMD and metadata table
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From: Randy Dunlap <rdunlap@infradead.org>
Date: 2026-10-03 21:44:49
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On 10/3/26 2:22 PM, Luigi Rizzo wrote:
quoted hunk ↗ jump to hunk
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