| 1 |
Jeffrey Gardner <je_fro@g.o> posted 463B6F3A.9050000@g.o, |
| 2 |
excerpted below, on Fri, 04 May 2007 12:36:58 -0500: |
| 3 |
|
| 4 |
> Processor type and features |
| 5 |
> [ ] IOMMU support |
| 6 |
|
| 7 |
Note that for AMD64, if you have >3.5 gig memory, you'll WANT IOMMU |
| 8 |
support, which uses the APGART hardware on AMD. On Intel, they don't |
| 9 |
have a hardware IOMMU but the kernel emulates it, using the same basic |
| 10 |
options, so I believe you'll want it there as well. |
| 11 |
|
| 12 |
Only four gig of memory is addressable from legacy 32-bit PCI devices, |
| 13 |
and there's a memory hole at the top of 4-gig memory (so beyond 3.5 gig) |
| 14 |
in ordered to allow device i/o memory access. With the correct BIOS |
| 15 |
settings, the machine will remap the unavailable memory behind that |
| 16 |
memory hole above 4 gig, but it and any memory you had beyond 4 gig |
| 17 |
already will not be directly accessible to DMA and the like from those |
| 18 |
legacy 32-bit PCI devices. IOMMU = input/output memory management unit. |
| 19 |
The hardware device maps high memory onto accessible addresses in the |
| 20 |
memory hole for the devices that need it, and of course the software |
| 21 |
emulation necessary for Intel machines does the same thing. |
| 22 |
|
| 23 |
Without that IOMMU, access to that > 4 gig area (because of the memory |
| 24 |
hole, to memory above ~3.5 gig) will be limited, and much slower for some |
| 25 |
devices if they work at all. Here, I simply cannot boot without IOMMU |
| 26 |
support (unless I disable part of my memory), as the kernel panics when |
| 27 |
it tries to read my hard drives. Apparently, either the SATA chipset |
| 28 |
they use or the kernel drivers supporting them are legacy 32-bit, and |
| 29 |
without the IOMMU, they simply cannot see the memory they are supposed to |
| 30 |
be DMAing stuff into. |
| 31 |
|
| 32 |
Of course, if you are still on legacy 32-bit x86 or have < 3.5 gig of |
| 33 |
memory (or are on a different arch entirely), the rules are somewhat |
| 34 |
different. I'm not sure how the IOMMU may be used there, or how much |
| 35 |
attempting to do without it might slow things down. |
| 36 |
|
| 37 |
-- |
| 38 |
Duncan - List replies preferred. No HTML msgs. |
| 39 |
"Every nonfree program has a lord, a master -- |
| 40 |
and if you use the program, he is your master." Richard Stallman |
| 41 |
|
| 42 |
-- |
| 43 |
gentoo-amd64@g.o mailing list |
Archives