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Documentation / Intel-IOMMU.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	Linux IOMMU Support
2	===================
3	
4	The architecture spec can be obtained from the below location.
5	
6	http://www.intel.com/technology/virtualization/
7	
8	This guide gives a quick cheat sheet for some basic understanding.
9	
10	Some Keywords
11	
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.
17	
18	Basic stuff
19	-----------
20	
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.
24	
25	What is RMRR?
26	-------------
27	
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.
34	
35	How is IOVA generated?
36	---------------------
37	
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.
41	
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.
46	
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.
50	
51	We also allocate guard pages with each mapping, so we can attempt to catch
52	any overflow that might happen.
53	
54	
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.
59	If this fixes anything, please ensure you file a bug reporting the problem.
60	
61	Some exceptions to IOVA
62	-----------------------
63	Interrupt ranges are not address translated, (0xfee00000 - 0xfeefffff).
64	The same is true for peer to peer transactions. Hence we reserve the
65	address from PCI MMIO ranges so they are not allocated for IOVA addresses.
66	
67	
68	Fault reporting
69	---------------
70	When errors are reported, the DMA engine signals via an interrupt. The fault
71	reason and device that caused it with fault reason is printed on console.
72	
73	See below for sample.
74	
75	
76	Boot Message Sample
77	-------------------
78	
79	Something like this gets printed indicating presence of DMAR tables
80	in ACPI.
81	
82	ACPI: DMAR (v001 A M I  OEMDMAR  0x00000001 MSFT 0x00000097) @ 0x000000007f5b5ef0
83	
84	When DMAR is being processed and initialized by ACPI, prints DMAR locations
85	and any RMRR's processed.
86	
87	ACPI DMAR:Host address width 36
88	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed90000
89	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed91000
90	ACPI DMAR:DRHD (flags: 0x00000001)base: 0x00000000fed93000
91	ACPI DMAR:RMRR base: 0x00000000000ed000 end: 0x00000000000effff
92	ACPI DMAR:RMRR base: 0x000000007f600000 end: 0x000000007fffffff
93	
94	When DMAR is enabled for use, you will notice..
95	
96	PCI-DMA: Using DMAR IOMMU
97	
98	Fault reporting
99	---------------
100	
101	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
102	DMAR:[fault reason 05] PTE Write access is not set
103	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
104	DMAR:[fault reason 05] PTE Write access is not set
105	
106	TBD
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.
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