20-24 September 2021
US/Pacific timezone

Shared Virtual Addressing (SVA) for in-kernel users

23 Sep 2021, 08:30
30m
Microconference2/Virtual-Room (LPC Virtual)

Microconference2/Virtual-Room

LPC Virtual

150
VFIO/IOMMU/PCI MC VFIO/IOMMU/PCI MC

Speaker

Jacob Pan

Description

Sharing virtual addresses between DMA and the user process is undoubtedly beneficial. It improves security by limiting DMA to the process virtual address space; The programming model is simplified by eliminating the need for explicit map/unmap operations with behind the scene IO page fault handling. Potential performance gains come after that.

However, applying the same logic to kernel-SVA is not without controversy. The DMA API is the de facto way of doing kernel DMA. It already provides portability and security by means of IOVA. DMA API is IOMMU agnostic and does not support IOMMU specific key concept of Process Address Space ID (PASID) which SVA relies on. IOVA is supported by IOMMU with separate page tables than the CPU counterpart.

In order to support SVA, IOMMU has to walk CPU page tables which undermines security if we allow sharing the entire kernel virtual address (KVA) space. IOTLB flush is also a gap since mmu_notifier is not available for kernel memory.

This proposed session explores the multiple candidates that can make DMA API compatible, KVA usage safe, and address the gap of IOTLB synchronization.

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Primary author

Jacob Pan

Presentation Materials