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

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Date:	Thu, 3 Sep 2015 19:59:02 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	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 Thu, Sep 03, 2015 at 08:17:02AM -0400, Prarit Bhargava wrote:
> 
> 
> On 09/02/2015 10:45 PM, Luis R. Rodriguez wrote:
> > On Mon, Aug 31, 2015 at 11:05:33AM -0500, Stuart Hayes wrote:
> >> Increase the range of chunk sizes tried in mtrr_cleanup() so it is able
> >> to map large memory configs into MTRRs.
> >>
> >> Currently, mtrr_cleanup() will fail with large memory configurations,
> >> because it limits chunk_size to 2GB, which means that each MTRR can only
> >> cover 2GB of memory.  With a memory size of, say, 256GB, and ten variable
> >> MTRRs (such as some recent Intel CPUs have), it is not possible to set up
> >> the MTRRs to cover all of memory.
> > 
> > Linux drivers no longer use MTRR so why is the cleanup needed, ie, what would
> > happen if the cleanup is just skipped in your case ?
> 
> The infiniband & video drivers still use MTRR (or at least it was my
> understanding that they do). 

There were a few stragglers left on v4.2, I have transformed them in the latest
development changes and those tranformations are now part of linux-next. If
this is specific to a driver you may want to first ensure you backport the
required patch that transforms the driver to use proper PAT interfaces, v4.2
should have most updates but there were still a few left. Just make sure your
driver doesn't call mtrr_add() directly and if it doesn't then you should be
OK.

> In any case, Stuart -- could you try booting with
> 'disable_mtrr_cleanup' as a kernel parameter?

Indeed, please I'd like to hear back. Be sure to have the respective driver
transformation in place, what driver are you using exactly? In the event that
you argue this is still needed I'd like to know exaclty *why*, the comit log
does not mention any of that at all.

  Luis
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