Re: x86_64 ten times slower than i386

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

Andi Kleen wrote:
>> Jesse Barnes (cc:d) wrote a patch to address this, I think (x86: trim
>> memory not covered by WB MTRRs), but as far as I can tell it hasn't
>> been merged yet. System is Intel, 4gb of RAM.
> 
> It wasn't merged because it broke booting on some systems.
> Besides the memory would be still lost -- all it did was to automate
> the "mem=XXXX" line.

There really are only two ways to deal with this -- drop the memory 
(which should be automated, and a warning printed) or adjust the MTRRs. 
  The problem is that at some point we run out of MTRRs, partially 
because they're masks instead of base/limit.

Even use of PAT doesn't trivially resolve this issue with less than 
doing MTRR emulation via PAT (setting the default MTRR to WB); however, 
that is bound to cause trouble with SMM.

	-hpa