Volker Armin Hemmann <volkerar...@googlemail.com> posted 200905061337.38907.volkerar...@googlemail.com, excerpted below, on Wed, 06 May 2009 13:37:38 +0200:
> On Mittwoch 06 Mai 2009, Nadav Horesh wrote: >> I recently added 4GB to the existing 2GB I had. The BIOS and lshw >> recognize the memory configuration (2 dimm of 1G and 2 of 2G), but >> /prc/meminfo and a test program I ran recognize 3.2GB. > get a non broken bios? ... Or possibly, reconfigure the BIOS you (OP) have. The story is this. Legacy PCI devices were designed for 32-bit memory only. That means their hardware I/O addresses must be below the 32-bit 4-gig memory ceiling. Back before memory got cheap and plentiful enough that people commonly ran 3+ gigs, that was no problem, the hardware addresses simply lived at the top of the 4-gig memory area, typically 3.5-4 gig. Now that common memory installations exceed that 3.5 gig barrier, the BIOS must be prepared to create a "memory hole", mapping memory that would otherwise fall behind the PCI hardware I/O area up beyond the 4-gig barrier. IOMMUs (input/output memory management units) or software bounce-buffers are then used to remap DMAs such that legacy PCI hardware operating with addresses in this memory hole can still handle DMAs working with memory addresses above the 4-gig barrier. This was in fact one of the reasons early AMD64 hardware was considered superior to early Intel em64t hardware, AMD had a GART based hardware IOMMU, while Intel hardware of the era required a software emulated IOMMU bounce-buffer, AMD hardware could thus handle DMAs directly, while Intel hardware had to emulate it in software. The kernel now has support for two additional forms of IOMMU, the newer AMD hardware and IBM's Calgary IOMMUs. I honestly don't know what modern Intel hardware uses, whether it's still software bounce-buffer based (the GART based IOMMU option includes the software emulator as well), or if they have a hardware IOMMU covered by one of the above kernel config options. If you've been in computers long enough to remember, the situation is somewhat analogous to the way early IBM PC hardware had conventional memory to 640 kB, a hardware memory area 640 kB to 1 MB, then expanded (early tech) or extended (later) memory beyond the 1 MB barrier. The 640 kB to 1 MB area was also called a "memory hole" with a BIOS option for handling it. If I'm not mistaken, it can still be seen in some modern BIOS in the form of an OS specifier, OS/2 or not. (It is however possible that I'm confused and that the OS/2 BIOS option is unrelated.) Anyway, check your BIOS. It's obviously difficult to refer to mine while I'm typing this, and I'm running AMD hardware (older first/second generation socket-940 Opteron, AMD 8xxx chipset) anyway, which may have different options or have it labeled differently, but look for an option mapping the memory hole as contiguous, discrete, or left undescribed, and try playing with that. If I recall correctly, my BIOS actually has two places to set something like that. I'm not sure how the two relate to each other nor do I recall precisely which each is labeled, but basically, when I upgraded memory from a gig to 8 gig, I played with these two BIOS options and the related kernel options until I got both BIOS and the kernel seeing the additional memory. Interestingly enough, the BIOS now counts to ~10 gig of memory, obviously counting the memory hole as well, but regardless, the kernel gets it right so I've left well enough alone since then. The related kernel config options I can actually check while I'm posting. =:^) These options can be found in menuconfig under processor type and features. As mentioned above, you'll want to figure out which IOMMU option matches your hardware and enable it. (The GART based option should work in any case, as it includes the software bounce-buffer emulation, but it may not be the fastest option if you have a hardware IOMMU OTHER than the original AMD 8xxx GART based version.) Then, look at memory model. Here, with current kernels, I have only one option, Sparse, evidently limited by my choice of hardware (Processor family and/or Supported processor vendor options, higher on the page, I'd guess or perhaps probed from the BIOS). However with older kernels, and presumably now if I were to choose hardware other than AMD Opteron/K8 (or choose a different BIOS option), there are Flat and Discontiguous options as well. Unfortunately, I can't tell you which options are correct for your (Intel) hardware. If you have multiple memory model choices, you may just have to try them and see. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman
