RE: [PATCH] Documentation: x86: add documenation for AMD IOMMU
From: Deucher, Alexander
Date: Wed Mar 09 2022 - 12:53:12 EST
[Public]
> -----Original Message-----
> From: Robin Murphy <robin.murphy@xxxxxxx>
> Sent: Tuesday, March 8, 2022 3:09 PM
> To: Deucher, Alexander <Alexander.Deucher@xxxxxxx>; linux-
> doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; corbet@xxxxxxx;
> hpa@xxxxxxxxx; x86@xxxxxxxxxx; dave.hansen@xxxxxxxxxxxxxxx;
> bp@xxxxxxxxx; mingo@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; joro@xxxxxxxxxx;
> Suthikulpanit, Suravee <Suravee.Suthikulpanit@xxxxxxx>; will@xxxxxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] Documentation: x86: add documenation for AMD
> IOMMU
>
> On 2022-03-08 19:04, Alex Deucher via iommu wrote:
> > Add preliminary documenation for AMD IOMMU.
> >
> > Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
> > ---
> > Documentation/x86/amd-iommu.rst | 85
> +++++++++++++++++++++++++++++++
> > Documentation/x86/index.rst | 1 +
> > Documentation/x86/intel-iommu.rst | 2 +-
> > 3 files changed, 87 insertions(+), 1 deletion(-)
> > create mode 100644 Documentation/x86/amd-iommu.rst
> >
> > diff --git a/Documentation/x86/amd-iommu.rst
> > b/Documentation/x86/amd-iommu.rst new file mode 100644 index
> > 000000000000..89820140fefa
> > --- /dev/null
> > +++ b/Documentation/x86/amd-iommu.rst
> > @@ -0,0 +1,85 @@
> > +=================
> > +AMD IOMMU Support
> > +=================
> > +
> > +The architecture spec can be obtained from the below location.
> > +
> >
> +https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fw
> ww
> >
> +.amd.com%2Fsystem%2Ffiles%2FTechDocs%2F48882_IOMMU.pdf&da
> ta=04%7C
> >
> +01%7Calexander.deucher%40amd.com%7C3adb51f8c3f1435e0deb08da013f
> 8172%7
> >
> +C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637823669974023501
> %7CUnkn
> >
> +own%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6I
> k1haWw
> >
> +iLCJXVCI6Mn0%3D%7C3000&sdata=9Wq07GM%2BdT9xt%2FCZ3xhue
> %2BrNIe6CnD
> > +cG32kwqosUEZ8%3D&reserved=0
> > +
> > +This guide gives a quick cheat sheet for some basic understanding.
> > +
> > +Some Keywords
> > +
> > +- IVRS - I/O Virtualization Reporting Structure
> > +- IVDB - I/O Virtualization Definition Block
> > +- IVHD - I/O Virtualization Hardware Definition
> > +- IOVA - I/O Virtual Address.
> > +
> > +Basic stuff
> > +-----------
> > +
> > +ACPI enumerates and lists the different DMA engines in the platform,
> > +and device scope relationships between PCI devices and which DMA
> > +engine controls them.
>
> "DMA engine" typically means a dedicated device for peripheral-to-memory
> or memory-to-memory transfers, or the responsible block within a general
> DMA-capable endpoint. In the context of the original Intel doc from whence I
> see this is copied, this probably should have said "DMAR unit"
> or similar; here I'd suggest picking your favourite vendor-appropriate term
> for "instance of IOMMU translation hardware". Let's not promote confusion
> more than necessary.
>
> > +
> > +What is IVRS?
> > +-------------
> > +
> > +The architecture defines an ACPI-compatible data structure called an
> > +I/O Virtualization Reporting Structure (IVRS) that is used to convey
> > +information related to I/O virtualization to system software. The
> > +IVRS describes the configuration and capabilities of the IOMMUs
> > +contained in the platform as well as information about the devices that
> each IOMMU virtualizes.
> > +
> > +The IVRS provides information about the following:
> > +- IOMMUs present in the platform including their capabilities and
> > +proper configuration
> > +- System I/O topology relevant to each IOMMU
> > +- Peripheral devices that cannot be otherwise enumerated
> > +- Memory regions used by SMI/SMM, platform firmware, and platform
> > +hardware. These are generally exclusion ranges to be configured by
> system software.
> > +
> > +How is IOVA generated?
> > +----------------------
> > +
> > +Well behaved drivers call pci_map_*() calls before sending command to
> > +device
>
> Horribly out-of-date drivers call pci_map_*(). Modern well-behaved drivers
> call dma_map_*() ;)
>
> > +that needs to perform DMA. Once DMA is completed and mapping is no
> > +longer required, device performs a pci_unmap_*() calls to unmap the
> region.
> > +
> > +The AMD IOMMU driver allocates a virtual address per domain. Each
> > +PCIE device has its own domain (hence protection). Devices under p2p
> > +bridges share the virtual address with all devices under the p2p
> > +bridge due to transaction id aliasing for p2p bridges.
> > +
> > +IOVA generation is pretty generic. We used the same technique as
> > +vmalloc() but these are not global address spaces, but separate for each
> domain.
> > +Different DMA engines may support different number of domains.
>
> I'm not sure about this whole section, really - IOVA management was entirely
> decoupled from drivers some time ago. If there's value in having some
> overview documentation, then it's probably worth consolidating into a
> common "IOMMU API" doc that can be cross-referenced for a summary of
> domains, groups, and iommu_dma_ops.
>
> > +
> > +
> > +Fault reporting
> > +---------------
> > +When errors are reported, the DMA engine signals via an interrupt. The
> fault
>
> Again, here I instinctively read "DMA engine" as being the endpoint
> device *making* the DMA transaction that faulted, and indeed that might
> happen to raise its own error interrupt if it gets an unexpected abort
> back from the IOMMU, which is coincidental to a thoroughly misleading
> degree...
Thanks for the review Robin. All good points. I've fixed them up and also fixed up the Intel documentation. Will send out v2 shortly.
Thanks,
Alex
>
> Thanks,
> Robin.
>
> > +reason and device that caused it with fault reason is printed on console.
> > +
> > +See below for sample.
> > +
> > +
> > +Boot Message Sample
> > +-------------------
> > +
> > +Something like this gets printed indicating presence of the IOMMU.
> > +
> > + iommu: Default domain type: Translated
> > + iommu: DMA domain TLB invalidation policy: lazy mode
> > +
> > +
> > +PCI-DMA: Using AMD IOMMU
> > +------------------------
> > +
> > +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]
> > +
> > diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> > index f498f1d36cd3..15711134eb68 100644
> > --- a/Documentation/x86/index.rst
> > +++ b/Documentation/x86/index.rst
> > @@ -22,6 +22,7 @@ x86-specific Documentation
> > mtrr
> > pat
> > intel-iommu
> > + amd-iommu
> > intel_txt
> > amd-memory-encryption
> > pti
> > diff --git a/Documentation/x86/intel-iommu.rst
> b/Documentation/x86/intel-iommu.rst
> > index 099f13d51d5f..4d3391c7bd3f 100644
> > --- a/Documentation/x86/intel-iommu.rst
> > +++ b/Documentation/x86/intel-iommu.rst
> > @@ -1,5 +1,5 @@
> > ===================
> > -Linux IOMMU Support
> > +Intel IOMMU Support
> > ===================
> >
> > The architecture spec can be obtained from the below location.