116 lines
3.9 KiB
ReStructuredText
116 lines
3.9 KiB
ReStructuredText
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===================
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Linux IOMMU Support
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===================
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The architecture spec can be obtained from the below location.
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http://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/vt-directed-io-spec.pdf
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This guide gives a quick cheat sheet for some basic understanding.
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Some Keywords
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- DMAR - DMA remapping
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- DRHD - DMA Remapping Hardware Unit Definition
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- RMRR - Reserved memory Region Reporting Structure
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- ZLR - Zero length reads from PCI devices
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- IOVA - IO Virtual address.
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Basic stuff
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-----------
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ACPI enumerates and lists the different DMA engines in the platform, and
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device scope relationships between PCI devices and which DMA engine controls
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them.
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What is RMRR?
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-------------
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There are some devices the BIOS controls, for e.g USB devices to perform
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PS2 emulation. The regions of memory used for these devices are marked
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reserved in the e820 map. When we turn on DMA translation, DMA to those
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regions will fail. Hence BIOS uses RMRR to specify these regions along with
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devices that need to access these regions. OS is expected to setup
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unity mappings for these regions for these devices to access these regions.
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How is IOVA generated?
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----------------------
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Well behaved drivers call pci_map_*() calls before sending command to device
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that needs to perform DMA. Once DMA is completed and mapping is no longer
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required, device performs a pci_unmap_*() calls to unmap the region.
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The Intel IOMMU driver allocates a virtual address per domain. Each PCIE
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device has its own domain (hence protection). Devices under p2p bridges
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share the virtual address with all devices under the p2p bridge due to
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transaction id aliasing for p2p bridges.
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IOVA generation is pretty generic. We used the same technique as vmalloc()
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but these are not global address spaces, but separate for each domain.
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Different DMA engines may support different number of domains.
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We also allocate guard pages with each mapping, so we can attempt to catch
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any overflow that might happen.
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Graphics Problems?
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------------------
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If you encounter issues with graphics devices, you can try adding
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option intel_iommu=igfx_off to turn off the integrated graphics engine.
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If this fixes anything, please ensure you file a bug reporting the problem.
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Some exceptions to IOVA
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-----------------------
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Interrupt ranges are not address translated, (0xfee00000 - 0xfeefffff).
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The same is true for peer to peer transactions. Hence we reserve the
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address from PCI MMIO ranges so they are not allocated for IOVA addresses.
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Fault reporting
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---------------
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When errors are reported, the DMA engine signals via an interrupt. The fault
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reason and device that caused it with fault reason is printed on console.
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See below for sample.
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Boot Message Sample
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-------------------
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Something like this gets printed indicating presence of DMAR tables
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in ACPI.
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ACPI: DMAR (v001 A M I OEMDMAR 0x00000001 MSFT 0x00000097) @ 0x000000007f5b5ef0
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When DMAR is being processed and initialized by ACPI, prints DMAR locations
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and any RMRR's processed::
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ACPI DMAR:Host address width 36
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ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed90000
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ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed91000
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ACPI DMAR:DRHD (flags: 0x00000001)base: 0x00000000fed93000
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ACPI DMAR:RMRR base: 0x00000000000ed000 end: 0x00000000000effff
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ACPI DMAR:RMRR base: 0x000000007f600000 end: 0x000000007fffffff
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When DMAR is enabled for use, you will notice..
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PCI-DMA: Using DMAR IOMMU
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-------------------------
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Fault reporting
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^^^^^^^^^^^^^^^
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::
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DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
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DMAR:[fault reason 05] PTE Write access is not set
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DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
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DMAR:[fault reason 05] PTE Write access is not set
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TBD
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----
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- For compatibility testing, could use unity map domain for all devices, just
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provide a 1-1 for all useful memory under a single domain for all devices.
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- API for paravirt ops for abstracting functionality for VMM folks.
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