Root/Documentation/Intel-IOMMU.txt

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

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