linux-kernel - Re: [tip:x86/mm] x86/mm/mtrr: Clean up mtrr_type

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Date:	Sat, 1 Aug 2015 09:39:07 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Toshi Kani <toshi.kani@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-mm <linux-mm@...ck.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/mm] x86/mm/mtrr: Clean up mtrr_type_lookup()

On Sat, Aug 1, 2015 at 9:33 AM, Borislav Petkov <bp@...en8.de> 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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