linux-kernel - Re: Fwd: [PATCH] x86: Use larger chunks in mtrr

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Date:	Thu, 03 Sep 2015 16:25:31 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Prarit Bhargava <prarit@...hat.com>,
	Stuart Hayes <stuart.w.hayes@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	mcgrof@...not-panic.com, Toshi Kani <toshi.kani@...com>
Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup

On Fri, 2015-09-04 at 00:07 +0200, Luis R. Rodriguez wrote:
 :
> These are still at odds, for instance, I was under the impression we can
> just have the OS return MTRR_TYPE_INVALID if the OS / drivers never used
> or set up MTRR, but the platform did, above (not the patch) you seem to be
> saying that even if the OS didn't modify MTRRs the OS still needs to return
> the appropriately set up MTRR type by firmware. This is different. Can you
> clarify?

mtrr_type_lookup() returns valid MTRR cache type for a given address range
when MTRRs are enabled.  It does not matter if MTRRs are set by the firmware
or the OS.  When MTRRs are enabled, the kernel needs to check through
mtrr_type_lookup() that large page mapping requests are aligned with MTRRs.

On Xen, or on a platform with firmware that does not enable MTRRs,
mtrr_type_lookup() returns MTRR_TYPE_INVALID (as long as the kernel does not
enable them).

Thanks,
-Toshi
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