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added the case when iommu needs to be disabled (#4037)

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* added the case when iommu needs to be disabled

* copy-pasta error
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spolifroni-amd
2024-11-21 12:31:55 -05:00
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parent ae5bf79b77
commit 92f20fc244
1 changed files with 14 additions and 6 deletions
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docs/conceptual/iommu.rst
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@@ -26,22 +26,30 @@ Linux supports IOMMU in both virtualized environments and bare metal.
The IOMMU is enabled by default but can be disabled or put into passthrough mode through the Linux kernel command line:
.. list-table::
:widths: 25 25 50
:widths: 15 15 70
:header-rows: 1
* - IOMMU Mode
- Kernel command
- Description
* - Enabled
- default setting
- The IOMMU is enabled in remapping mode. Each device gets its own I/O virtual address space. All devices on Linux register their DMA addressing capabilities, and the kernel will ensure that any address space mapped for DMA is mapped within the device's DMA addressing limits. Only address space explicitly mapped by the devices will be mapped into virtual address space. Attempts to access an unmapped page will generate an IOMMU page fault. This setting is recommended for AMD Radeon GPUs that need peer-to-peer DMA.
- Default setting
- Recommended for AMD Radeon GPUs that need peer-to-peer DMA.
The IOMMU is enabled in remapping mode. Each device gets its own I/O virtual address space. All devices on Linux register their DMA addressing capabilities, and the kernel will ensure that any address space mapped for DMA is mapped within the device's DMA addressing limits. Only address space explicitly mapped by the devices will be mapped into virtual address space. Attempts to access an unmapped page will generate an IOMMU page fault.
* - Passthrough
- ``iommu=pt``
- Interrupt remapping is enabled but I/O remapping is disabled. The entire platform shares a common platform address space for system memory and MMIO spaces, ensuring compatibility with drivers from external vendors, while still supporting CPUs with a large number of cores. This setting is recommended for AMD Instinct Accelerators and for AMD Radeon GPUs that don't need peer-to-peer DMA.
- Recommended for AMD Instinct Accelerators and for AMD Radeon GPUs that don't need peer-to-peer DMA.
Interrupt remapping is enabled but I/O remapping is disabled. The entire platform shares a common platform address space for system memory and MMIO spaces, ensuring compatibility with drivers from external vendors, while still supporting CPUs with a large number of cores.
* - Disabled
- ``iommu=off``
- The IOMMU is disabled and the entire platform shares a common platform address space for system memory and MMIO spaces.
- Not recommended.
The IOMMU is disabled and the entire platform shares a common platform address space for system memory and MMIO spaces.
This mode should only be used with older Linux distributions with kernels that are not configured to support peer-to-peer DMA with an IOMMU. In these cases, the IOMMU needs to be disabled to use peer-to-peer DMA.
The IOMMU also provides virtualized access to the MMIO portions of the platform address space for peer-to-peer DMA.
Because peer-to-peer DMA is not officially part of the PCI/PCIe specification, the behavior of peer-to-peer DMA varies between hardware platforms.