On Mon, Jul 11, 2005 at 06:27:56PM +0200, Dieter St?ken wrote:
in combination: First I learned, that Asus boards with a VIA chipset have some serious hardware problems dealing with >= 4Gb of memory. The Bios offered some switch to map any memory in conflict to regions above 4G, but the newest Bios update misses this option, as it did not work stable.
It should still work even without remapping, just you lose some memory where the IO hole is mapped over it. What might not work and often indeed doesn't is to enable the mapping option.
So Q1: does it work in any way with 2.6.12 (i installed now) by setting any iommu=??? Why is addressing above 4gb a problem for a 64-bit cpu, anyway?
The CPU has no problems with that, just some devices on your motherboard. If you want to truly escape the 4gb limit you need 64bit DMA capable devices too. e.g. most SCSI controllers support that, most IDE controllers don't.
I bought this 64bit system exactly to get rid of any ugly 4gb limit :-( I got messages like: PCI-DMA: "Out of SW-IOMMU space ..." but I'm not sure about what iommu setting I used for this try.
Some device needed more than 64MB of bounce buffer. On VIA use swiotlb=32768 or swiotlb=65536 For some reason nobody understands the standard IOMMU doesn't work on VIA so the software bounce buffering is used.
Problem 2 seems to be a serious stability problem when pug in all 4G (4x1G Kingston registered ECC modules). My bios reports spurious strange "SYSTEM FAILURE DUE TO CPU OVERCLOCKING" messages, even if I did not change any settings at all. After several attempts to boot, the system even stays black.
Doesn't sound like a Linux related problem. -Andi