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

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Date:	Thu, 3 Sep 2015 21:51:34 +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>
Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup

On Thu, Sep 03, 2015 at 01:22:42PM -0600, Toshi Kani wrote:
> On Thu, 2015-09-03 at 20:40 +0200, Luis R. Rodriguez wrote:
> > On Thu, Sep 03, 2015 at 02:10:14PM -0400, Prarit Bhargava wrote:
> > > 
> > > 
> > > On 09/03/2015 01:59 PM, Luis R. Rodriguez wrote:
> > > > 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.
> > > > 
> > > 
> > > Well ... we are trying to also fix this in older kernels too, *cough* RHEL
> > > *cough*, so that's where the patch comes from.  If upstream is going to
> > > deprecate/remove mtrr support so be it. 
> > 
> > Check linux-next, and Documentation/x86/mtrr.txt
> > 
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/Docume
> > ntation/x86/mtrr.txt
> > 
> > The platform use of MTRR is the only thing you should be concerned over but 
> > as noted returning MTRR_TYPE_INVALID should suffice if the OS does not make
> > any driver use / modifications.
> 
> The following sentence in the "mtrr.txt" is not correct.  (Sorry I should have
> caught it earlier.)  mtrr_type_lookup() returns MTRR_TYPE_INVALID when MTRRs
> are disabled, i.e. MTRRs are not set by neither firmware nor OS.  Most of the
> firmwares enable them, though.
>
> "If MTRRs are only set up by the platform firmware code though and the OS does
> not make any specific MTRR mapping requests mtrr_type_lookup() should always
> return MTRR_TYPE_INVALID."

So this should be clarified to say -- that because platform firmware *may* make
use of MTRRs, even if all OS drivers are no longer making use of MTRRs
directly, we still need mtrr_type_lookup() to return the right type ?

 Luis
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