[coreboot] MTRR setup strategy

Stefan Reinauer stepan at coresystems.de
Sat Jan 24 20:58:01 CET 2009

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.


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