linux-kernel - Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"

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Date:	Mon, 7 Mar 2016 18:53:28 -0500
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Toshi Kani <toshi.kani@....com>
CC:	Borislav Petkov <bp@...e.de>,
	Richard Purdie <richard.purdie@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	Bruce Ashfield <bruce.ashfield@...driver.com>,
	"Hart, Darren" <darren.hart@...el.com>,
	"saul.wold" <saul.wold@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"

[Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"] On 07/03/2016 (Mon 16:38) Toshi Kani wrote:

> On Mon, 2016-03-07 at 16:08 -0500, Paul Gortmaker wrote:
> > [dropping oe list and lkml since attaching dmesg files.]
> > 

[...]

> > > Yes, please send me full dmesg files.  Since I do not know your
> > > original state, the diff does not give me the whole picture.
> > 
> > Attached.
> 
> Thanks for the dmesg files!  As I suspected, there is no message from
> pat_init() in both cases.  That is, you are missing the following message,
> which shows how PAT is configured to support cache attributes.
> 
> # dmesg | grep PAT
> [0.000000] x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WC  UC- WT  

Interesting...

> 
> It may have seemed working before, but you did not have WC configured to
> PAT without calling pat_init().  There was not a proper check in place to
> detect this error before.  Can you please check your code to see what
> caused this skip of pat_init()?  If you have a git tree, I can take a look
> as well. 

You already have git copies of what I'm running, since it is vanilla
mainline commits.  No code changes at this end whatsoever.  I did the
bisect on vanilla mainline.  All I took from yocto was their ".config"

To recap, v4.1-rc5-21-g9dac62909451 works,  v4.1-rc5-22-g9cd25aac1f44
fails, and v4.5-rc6 also fails.  If pat_init() isn't called then this
is a bug in current mainline.  I'll have a look later myself and see
if I can trace out how we expect to get to pat_init() and how that
might be skipped inadvertently unless someone beats me to it.

Paul.
--

> 
> -Toshi