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

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Date:	Fri, 4 Sep 2015 00:45:56 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Toshi Kani <toshi.kani@....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>,
	Jan Beulich <JBeulich@...e.com>,
	Juergen Gross <jgross@...e.com>
Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup

On Thu, Sep 03, 2015 at 04:25:31PM -0600, Toshi Kani wrote:
> 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.

One further change I was considering was seeing if we can separate PAT
set up from MTRR's setup, but that was under the assumption we could live
with a kernel that would have mtrr_type_lookup() return MTRR_TYPE_INVALID
if kernel MTRR code is completely disabled but PAT enabled. We can't enable PAT
today without MTRR beceause PAT is initialized from the MTRR init sequence and
that depends on MTRR, if we separated these though and and if a distro disabled
kernel MTRR an but enabled PAT and if MTRR did set up MTRR what would the possible
issues be?

> On Xen,

When Xen is used a platform firmware may still set up MTRR, even if the
hypervisor doesn't set up MTRR right ? So same issue and question here.

> 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).

Sure this makes sense.

Thanks in advance,

  Luis
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