Re: PAT support
From: Eric W. Biederman
Date: Thu Apr 15 2004 - 13:49:07 EST
Andi Kleen <ak@xxxxxx> 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
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