Re: PAT support

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <ak@muc.de> writes:

> > This would also be extremely useful on machines with large amounts
> > of memory, for write-back mappings.  With large amounts but odd sized
> > entries it becomes extremely tricky to map all of the memory using
> > mtrrs.
> 
> Yes agreed. I already had vendors complaining about this.
> But for this it will need some more work - the MTRRs need to be fully
> converted to PAT and then disabled (because MTRRs have 
> higher priority than PAT). Doing so is a lot more risky than 
> what Terrence's patch does currently though.  But longer term
> we will need it.

Ugh.  You are right.  The processors look at the two types and pick
the one that caches the least.  So PAT can't enable caching :(

> Also it will still need to handle overlapping ranges. I suppose
> it will need some simple rules like: converting from UC to WC is 
> always ok.

Right.

That plus it should have some additional rules like the
e820 map trumps the mtrrs in specifying what is memory so
should be cacheable.

Eric