1===================
2Linux IOMMU Support
3===================
4
5The architecture spec can be obtained from the below location.
6
7http://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/vt-directed-io-spec.pdf
8
9This guide gives a quick cheat sheet for some basic understanding.
10
11Some Keywords
12
13- DMAR - DMA remapping
14- DRHD - DMA Remapping Hardware Unit Definition
15- RMRR - Reserved memory Region Reporting Structure
16- ZLR  - Zero length reads from PCI devices
17- IOVA - IO Virtual address.
18
19Basic stuff
20-----------
21
22ACPI enumerates and lists the different DMA engines in the platform, and
23device scope relationships between PCI devices and which DMA engine  controls
24them.
25
26What is RMRR?
27-------------
28
29There are some devices the BIOS controls, for e.g USB devices to perform
30PS2 emulation. The regions of memory used for these devices are marked
31reserved in the e820 map. When we turn on DMA translation, DMA to those
32regions will fail. Hence BIOS uses RMRR to specify these regions along with
33devices that need to access these regions. OS is expected to setup
34unity mappings for these regions for these devices to access these regions.
35
36How is IOVA generated?
37----------------------
38
39Well behaved drivers call pci_map_*() calls before sending command to device
40that needs to perform DMA. Once DMA is completed and mapping is no longer
41required, device performs a pci_unmap_*() calls to unmap the region.
42
43The Intel IOMMU driver allocates a virtual address per domain. Each PCIE
44device has its own domain (hence protection). Devices under p2p bridges
45share the virtual address with all devices under the p2p bridge due to
46transaction id aliasing for p2p bridges.
47
48IOVA generation is pretty generic. We used the same technique as vmalloc()
49but these are not global address spaces, but separate for each domain.
50Different DMA engines may support different number of domains.
51
52We also allocate guard pages with each mapping, so we can attempt to catch
53any overflow that might happen.
54
55
56Graphics Problems?
57------------------
58If you encounter issues with graphics devices, you can try adding
59option intel_iommu=igfx_off to turn off the integrated graphics engine.
60If this fixes anything, please ensure you file a bug reporting the problem.
61
62Some exceptions to IOVA
63-----------------------
64Interrupt ranges are not address translated, (0xfee00000 - 0xfeefffff).
65The same is true for peer to peer transactions. Hence we reserve the
66address from PCI MMIO ranges so they are not allocated for IOVA addresses.
67
68
69Fault reporting
70---------------
71When errors are reported, the DMA engine signals via an interrupt. The fault
72reason and device that caused it with fault reason is printed on console.
73
74See below for sample.
75
76
77Boot Message Sample
78-------------------
79
80Something like this gets printed indicating presence of DMAR tables
81in ACPI.
82
83ACPI: DMAR (v001 A M I  OEMDMAR  0x00000001 MSFT 0x00000097) @ 0x000000007f5b5ef0
84
85When DMAR is being processed and initialized by ACPI, prints DMAR locations
86and any RMRR's processed::
87
88	ACPI DMAR:Host address width 36
89	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed90000
90	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed91000
91	ACPI DMAR:DRHD (flags: 0x00000001)base: 0x00000000fed93000
92	ACPI DMAR:RMRR base: 0x00000000000ed000 end: 0x00000000000effff
93	ACPI DMAR:RMRR base: 0x000000007f600000 end: 0x000000007fffffff
94
95When DMAR is enabled for use, you will notice..
96
97PCI-DMA: Using DMAR IOMMU
98
99Fault reporting
100---------------
101
102::
103
104	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
105	DMAR:[fault reason 05] PTE Write access is not set
106	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
107	DMAR:[fault reason 05] PTE Write access is not set
108
109TBD
110----
111
112- For compatibility testing, could use unity map domain for all devices, just
113  provide a 1-1 for all useful memory under a single domain for all devices.
114- API for paravirt ops for abstracting functionality for VMM folks.
115