> Also, note that the previous help text stated that IOMMU was needed for
> >3GB memory instead of >4GB. This is fixed in the newer version.
note that many system bioses have memory remapping, to accomodate pci
i/o ranges. some address space is reserved by the bios for these i/o
ranges, and as system memory approaches this reserved space, the
memory is remapped to >4GB. this usually happens around 3.25GB -
3.5GB, but probably varies based on bios and pci devices. once this
memory is remapped to >4GB, the IOMMU kicks in.
so the original text is probably more accurate.
On Wed, Mar 08, 2006 at 03:48:30PM -0600, [email protected] wrote:
> Oops, forgot to CC lkml.
>
> ----- Forwarded message from Jon Mason <[email protected]> -----
>
> User-Agent: Mutt/1.5.11
> From: Jon Mason <[email protected]>
> To: Andi Kleen <[email protected]>
> Date: Wed, 8 Mar 2006 15:45:49 -0600
> Subject: [PATCH] x86-64: Make GART_IOMMU kconfig help text more specific (trivial)
>
> Have the GART_IOMMU help text specify that this is the hardware IOMMU in
> amd64 processors. This will be significant if/when other IOMMUs are
> added to the x86-64 architecture. :-)
>
> Also, note that the previous help text stated that IOMMU was needed for
> >3GB memory instead of >4GB. This is fixed in the newer version.
>
> Thanks,
> Jon
>
> Signed-off-by: Jon Mason <[email protected]>
>
> diff -r 149aa2a22913 arch/x86_64/Kconfig
> --- a/arch/x86_64/Kconfig Tue Feb 28 22:02:10 2006
> +++ b/arch/x86_64/Kconfig Wed Mar 8 15:24:44 2006
> @@ -364,13 +364,14 @@
> select SWIOTLB
> depends on PCI
> help
> - Support the IOMMU. Needed to run systems with more than 3GB of memory
> - properly with 32-bit PCI devices that do not support DAC (Double Address
> - Cycle). The IOMMU can be turned off at runtime with the iommu=off parameter.
> - Normally the kernel will take the right choice by itself.
> - This option includes a driver for the AMD Opteron/Athlon64 northbridge IOMMU
> - and a software emulation used on other systems.
> - If unsure, say Y.
> + Support for hardware IOMMU in AMD's Opteron/Athlon64 Processors.
> + Needed to run systems with more than 4GB of memory properly with
> + 32-bit PCI devices that do not support DAC (Double Address Cycle).
> + The IOMMU can be turned off at runtime with the iommu=off parameter.
> + Normally the kernel will take the right choice by itself.
> + This option includes a driver for the AMD Opteron/Athlon64 IOMMU
> + northbridge and a software emulation used on some other systems.
> + If unsure, say Y.
>
> # need this always enabled with GART_IOMMU for the VIA workaround
> config SWIOTLB
>
> ----- End forwarded message -----
> -
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