Re: [tip:x86/mm] x86/mm/mtrr: Clean up mtrr_type_lookup()

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Sat, Aug 1, 2015 at 9:33 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> Well, it doesn't really make sense to write-combine IO memory, does it?

Quite the reverse.

It makes no sense to write-combine normal memory (RAM), because caches
work and sane memory is always cache-coherent. So marking regular
memory write-combining is a sign of crap hardware (which admittedly
exists all too much, but hopefully goes away).

In contrast, marking MMIO memory write-combining is not a sign of crap
hardware - it's just a sign of things like frame buffers on the card
etc. Which very much wants write combining. So WC for MMIO at least
makes sense.

Yes, yes, I realize that "crap hardware" may actually be the more
common case, but still..

             Linus
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