On Wed, Dec 13, 2006 at 09:34:16PM +0100, Karsten Weiss wrote:
> FWIW: As far as I understand the linux kernel code (I am no kernel
> developer so please correct me if I am wrong) the PCI dma mapping code is
> abstracted by struct dma_mapping_ops. I.e. there are currently four
> possible implementations for x86_64 (see
> linux-2.6/arch/x86_64/kernel/)
>
> 1. pci-nommu.c : no IOMMU at all (e.g. because you have < 4 GB memory)
> Kernel boot message: "PCI-DMA: Disabling IOMMU."
>
> 2. pci-gart.c : (AMD) Hardware-IOMMU.
> Kernel boot message: "PCI-DMA: using GART IOMMU" (this message
> first appeared in 2.6.16)
>
> 3. pci-swiotlb.c : Software-IOMMU (used e.g. if there is no hw iommu)
> Kernel boot message: "PCI-DMA: Using software bounce buffering
> for IO (SWIOTLB)"
Used if there's no HW IOMMU *and* it's needed (because you have >4GB
memory) or you told the kernel to use it (iommu=soft).
> 4. pci-calgary.c : Calgary HW-IOMMU from IBM; used in pSeries servers.
> This HW-IOMMU supports dma address mapping with memory proctection,
> etc.
> Kernel boot message: "PCI-DMA: Using Calgary IOMMU" (since
> 2.6.18!)
Calgary is found in pSeries servers, but also in high-end xSeries
(Intel based) servers. It would be a little awkward if pSeries servers
(which are based on PowerPC processors) used code under arch/x86-64
:-)
> BTW: It would be really great if this area of the kernel would get some
> more and better documentation. The information at
> linux-2.6/Documentation/x86_64/boot_options.txt is very terse. I had to
> read the code to get a *rough* idea what all the "iommu=" options
> actually do and how they interact.
Patches happily accepted :-)
Cheers,
Muli
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