linux-kernel - Re: [PATCH v4] Documentation: x86: rework IOMMU documentation

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Date:   Fri, 22 Apr 2022 20:41:07 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Alex Deucher <alexander.deucher@....com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        corbet@....net, hpa@...or.com, x86@...nel.org,
        dave.hansen@...ux.intel.com, bp@...en8.de, mingo@...hat.com,
        tglx@...utronix.de, joro@...tes.org, Suravee.Suthikulpanit@....com,
        will@...nel.org, iommu@...ts.linux-foundation.org,
        Vasant.Hegde@....com
Subject: Re: [PATCH v4] Documentation: x86: rework IOMMU documentation

On 2022-04-22 18:54, Alex Deucher wrote:
[...]
> +Intel Specific Notes
> +--------------------
> +
> +Graphics Problems?
> +^^^^^^^^^^^^^^^^^^
> +
> +If you encounter issues with graphics devices, you can try adding
> +option intel_iommu=igfx_off to turn off the integrated graphics engine.
> +If this fixes anything, please ensure you file a bug reporting the problem.
> +
> +Some exceptions to IOVA
> +^^^^^^^^^^^^^^^^^^^^^^^
> +
> +Interrupt ranges are not address translated, (0xfee00000 - 0xfeefffff).
> +The same is true for peer to peer transactions. Hence we reserve the
> +address from PCI MMIO ranges so they are not allocated for IOVA addresses.

Note that this should be true for both drivers.

> +
> +AMD Specific Notes
> +------------------
> +
> +Graphics Problems?
> +^^^^^^^^^^^^^^^^^^
> +
> +If you encounter issues with integrated graphics devices, you can try adding
> +option iommu=pt to the kernel command line use a 1:1 mapping for the IOMMU.  If
> +this fixes anything, please ensure you file a bug reporting the problem.

And indeed this is a generic option. I reckon we could simply merge 
these two sections together, with the first paragraph being something like:

If you encounter issues with integrated graphics devices, you can try 
adding the option "iommu.passthrough=1", or the equivalent "iommu=pt", 
to the kernel command line to use a 1:1 mapping for the IOMMU in 
general.  On Intel you can also try "intel_iommu=igfx_off" to turn off 
translation specifically for the integrated graphics engine only.  If 
this fixes anything, please ensure you file a bug reporting the problem.

> +
> +Fault reporting
> +---------------
> +When errors are reported, the IOMMU signals via an interrupt. The fault
> +reason and device that caused it is printed on the console.
> +
> +
> +Kernel Log Samples
> +------------------
> +
> +Intel Boot Messages
> +^^^^^^^^^^^^^^^^^^^
> +
> +Something like this gets printed indicating presence of DMAR tables
> +in ACPI.
> +
> +::
> +
> +	ACPI: DMAR (v001 A M I  OEMDMAR  0x00000001 MSFT 0x00000097) @ 0x000000007f5b5ef0
> +
> +When DMAR is being processed and initialized by ACPI, prints DMAR locations
> +and any RMRR's processed
> +
> +::
> +
> +	ACPI DMAR:Host address width 36
> +	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed90000
> +	ACPI DMAR:DRHD (flags: 0x00000000)base: 0x00000000fed91000
> +	ACPI DMAR:DRHD (flags: 0x00000001)base: 0x00000000fed93000
> +	ACPI DMAR:RMRR base: 0x00000000000ed000 end: 0x00000000000effff
> +	ACPI DMAR:RMRR base: 0x000000007f600000 end: 0x000000007fffffff
> +
> +When DMAR is enabled for use, you will notice
> +
> +::
> +
> +	PCI-DMA: Using DMAR IOMMU
> +
> +Intel Fault reporting
> +^^^^^^^^^^^^^^^^^^^^^
> +
> +::
> +
> +	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
> +	DMAR:[fault reason 05] PTE Write access is not set
> +	DMAR:[DMA Write] Request device [00:02.0] fault addr 6df084000
> +	DMAR:[fault reason 05] PTE Write access is not set
> +
> +AMD Boot Messages
> +^^^^^^^^^^^^^^^^^
> +
> +Something like this gets printed indicating presence of the IOMMU.
> +
> +::
> +
> +	iommu: Default domain type: Translated
> +	iommu: DMA domain TLB invalidation policy: lazy mode

Similarly, that's common IOMMU API reporting which will be seen on all 
architectures (let alone IOMMU drivers). Maybe some of the messages from 
print_iommu_info() might be better AMD-specific examples?

Cheers,
Robin.

> +
> +AMD Fault reporting
> +^^^^^^^^^^^^^^^^^^^
> +
> +::
> +
> +	AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0007 address=0xffffc02000 flags=0x0000]
> +	AMD-Vi: Event logged [IO_PAGE_FAULT device=07:00.0 domain=0x0007 address=0xffffc02000 flags=0x0000]