Re: Setting up MTRRs for 4096MB RAM

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <Pine.LNX.4.21.0108091306550.18150-100000@willow.commerce.uk.net>
By author:    Corin Hartland-Swann <cdhs@commerce.uk.net>
In newsgroup: linux.dev.kernel
> 
> I am trying to set up a machine using the Tyan Tiger LE motherboard, and
> ServerWorks III LE chipset to use 4096MB RAM. I'm using kernel 2.4.7 with
> CONFIG_HIGHMEM4G.
> 
> I know that I have to set the MTRR's up to extend the cacheable memory
> area, but can't work out how to set it up.
> 
> I tried the following:
> 
>   # echo "disable=1" >| /proc/mtrr
>   # echo "disable=0" >| /proc/mtrr
>   # echo "base=0x0 size=0xFFFFFFFF type=write-back" >| /proc/mtrr
>   mtrr: size and base must be multiples of 4 kiB
>   mtrr: size: 0xffffffff  base: 0x0
> 
> Which doesn't make any sense. So I tried for 3G RAM:
> 
>   # echo "base=0x0 size=0xC0000000 type=write-back" >| /proc/mtrr
>   mtrr: base(0x0000) is not aligned on a size(0xc0000000) boundary
> 
> And then for 2G RAM:
> 
>   # echo "base=0x0 size=0x80000000 type=write-back" >| /proc/mtrr
> 
> Which works perfectly. What gives?
> 

Intel MTRRs have to be a multiple of 2, so you'd need 2 MTRRs if you
wanted to cover 3 GB.  0x80000000 is a multiple of 2; 0xC0000000
isn't, and 0xFFFFFFFF definitely isn't, although 0x100000000 is.

I'm surprised you didn't see that very pattern in your own responses:
you are entering the full size in the working case, but then subtract
1 in the 4 GB case.

	-hpa
-- 
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