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Guest on 26th May 2021 07:37:08 PM

  1. Linux IOMMU Support
  2. ===================
  4. The architecture spec can be obtained from the below location.
  6. http://www.intel.com/technology/virtualization/
  8. This guide gives a quick cheat sheet for some basic understanding.
  10. Some Keywords
  12. DMAR - DMA remapping
  13. DRHD - DMA Engine Reporting Structure
  14. RMRR - Reserved memory Region Reporting Structure
  15. ZLR  - Zero length reads from PCI devices
  16. IOVA - IO Virtual address.
  18. Basic stuff
  19. -----------
  21. ACPI enumerates and lists the different DMA engines in the platform, and
  22. device scope relationships between PCI devices and which DMA engine  controls
  23. them.
  25. What is RMRR?
  26. -------------
  28. There are some devices the BIOS controls, for e.g USB devices to perform
  29. PS2 emulation. The regions of memory used for these devices are marked
  30. reserved in the e820 map. When we turn on DMA translation, DMA to those
  31. regions will fail. Hence BIOS uses RMRR to specify these regions along with
  32. devices that need to access these regions. OS is expected to setup
  33. unity mappings for these regions for these devices to access these regions.
  35. How is IOVA generated?
  36. ---------------------
  38. Well behaved drivers call pci_map_*() calls before sending command to device
  39. that needs to perform DMA. Once DMA is completed and mapping is no longer
  40. required, device performs a pci_unmap_*() calls to unmap the region.
  42. The Intel IOMMU driver allocates a virtual address per domain. Each PCIE
  43. device has its own domain (hence protection). Devices under p2p bridges
  44. share the virtual address with all devices under the p2p bridge due to
  45. transaction id aliasing for p2p bridges.
  47. IOVA generation is pretty generic. We used the same technique as vmalloc()
  48. but these are not global address spaces, but separate for each domain.
  49. Different DMA engines may support different number of domains.
  51. We also allocate gaurd pages with each mapping, so we can attempt to catch
  52. any overflow that might happen.
  55. Graphics Problems?
  56. ------------------
  57. If you encounter issues with graphics devices, you can try adding
  58. option intel_iommu=igfx_off to turn off the integrated graphics engine.
  60. If it happens to be a PCI device included in the INCLUDE_ALL Engine,
  61. then try enabling CONFIG_DMAR_GFX_WA to setup a 1-1 map. We hear
  62. graphics drivers may be in process of using DMA api's in the near
  63. future and at that time this option can be yanked out.
  65. Some exceptions to IOVA
  66. -----------------------
  67. Interrupt ranges are not address translated, (0xfee00000 - 0xfeefffff).
  68. The same is true for peer to peer transactions. Hence we reserve the
  69. address from PCI MMIO ranges so they are not allocated for IOVA addresses.
  72. Fault reporting
  73. ---------------
  74. When errors are reported, the DMA engine signals via an interrupt. The fault
  75. reason and device that caused it with fault reason is printed on console.
  77. See below for sample.
  80. Boot Message Sample
  81. -------------------
  83. Something like this gets printed indicating presence of DMAR tables
  84. in ACPI.
  86. ACPI: DMAR (v001 A M I  OEMDMAR  0x00000001 MSFT 0x00000097) @ 0x000000007f5b5ef0
  88. When DMAR is being processed and initialized by ACPI, prints DMAR locations
  89. and any RMRR's processed.
  91. ACPI DMAR:Host address width 36
  92. ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed90000
  93. ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed91000
  94. ACPI DMAR:DRHD (flags: 0x00000001)base: 0x00000000fed93000
  95. ACPI DMAR:RMRR base: 0x00000000000ed000 end: 0x00000000000effff
  96. ACPI DMAR:RMRR base: 0x000000007f600000 end: 0x000000007fffffff
  98. When DMAR is enabled for use, you will notice..
  100. PCI-DMA: Using DMAR IOMMU
  102. Fault reporting
  103. ---------------
  105. DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
  106. DMAR:[fault reason 05] PTE Write access is not set
  107. DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
  108. DMAR:[fault reason 05] PTE Write access is not set
  110. TBD
  111. ----
  113. - For compatibility testing, could use unity map domain for all devices, just
  114.   provide a 1-1 for all useful memory under a single domain for all devices.
  115. - API for paravirt ops for abstracting functionlity for VMM folks.

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