Carl-Daniel Hailfinger wrote: > Example: > We want to cache 0MB - (2G-64M-64k). > Where do the 64k come from? > Current setup: > reg00: base=0x00000000 ( 0MB), size=1024MB: write-back, count=1 > reg01: base=0x40000000 (1024MB), size= 512MB: write-back, count=1 > reg02: base=0x60000000 (1536MB), size= 256MB: write-back, count=1 > reg03: base=0x70000000 (1792MB), size= 128MB: write-back, count=1 > reg04: base=0x78000000 (1920MB), size= 32MB: write-back, count=1 > reg05: base=0x7a000000 (1952MB), size= 16MB: write-back, count=1 > reg06: base=0x7b000000 (1968MB), size= 8MB: write-back, count=1 > reg07: base=0x7b800000 (1976MB), size= 4MB: write-back, count=1 > --Here we run out of MTRRs, additionally needed MTRRs follow-- > And actually we should have run out of MTRRs 2 steps earlier. The BIOS is only allowed to grab 6 of the 8 possible MTRRs. The other two have to be left free to use by the operating system. There is code in coreboot to enforce this, but it's #if 0'ed. > We could achieve the same effect with a subtractive setup: > reg00: base=0x00000000 ( 0MB), size=2048MB: write-back, count=1 > reg01: base=0x7c000000 (1984MB), size=64MB: uncached, count=1 > reg02: base=0x7bff0000 (1983MB), size=64kB: uncached, count=1 > > However, a subtractive setup is not always more efficient. Is it not? It sounds like at least if we have 2^x bytes of memory and subtract a small chunk or two, we would be quite well off with it.
I wonder... the subtractive strategy you subscribe is mentioned in
mtrr.c too and the comment claims it is implemented. But it very much
seems it is not.
> That means we
> have to select the best setup type. I devised a slightly tricky
> algorithm to do that:
> 1. Check if there are multiple disjoint cached areas in a given
> power-of-two sized area.
> 1a. If no, go to step 2
> 1b. If yes, stop here. Need advanced setup not described here.
>
please describe ;)
> 2. additive_count=bitcount(top_cached_addr+1)
> 3.
> subtractive_count=bitcount(rounduptonextpowerof2(top_cached_addr)-(top_cached_addr+1))
> 4. if (additive_count>subtractive_count) go to subtractive_method else
> go to additive_method
>
Yes, sounds good.
Stefan
--
coresystems GmbH • Brahmsstr. 16 • D-79104 Freiburg i. Br.
Tel.: +49 761 7668825 • Fax: +49 761 7664613
Email: [email protected] • http://www.coresystems.de/
Registergericht: Amtsgericht Freiburg • HRB 7656
Geschäftsführer: Stefan Reinauer • Ust-IdNr.: DE245674866
signature.asc
Description: OpenPGP digital signature
-- coreboot mailing list: [email protected] http://www.coreboot.org/mailman/listinfo/coreboot
