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Message-ID: <1457400913.15454.435.camel@hpe.com>
Date: Mon, 07 Mar 2016 18:35:13 -0700
From: Toshi Kani <toshi.kani@....com>
To: Paul Gortmaker <paul.gortmaker@...driver.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"
On Mon, 2016-03-07 at 17:56 -0700, Toshi Kani wrote:
> On Mon, 2016-03-07 at 18:53 -0500, Paul Gortmaker wrote:
> > [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.
>
> Oh, I see. Can you send me the ".config" file?
And also an output of /proc/cpuinfo, please?
I think I know what's going on. I noticed that you have the following
message in your dmesg files.
[ 0.000000] MTRR: Disabled
MTRR is set to disabled when your CPU is Intel but does not support MTRR.
Perhaps, QEMU does not emulate MTRR?
pat_init() is not called when MTRR is disabled. I think this dependency is
wrong, and it needs to be fixed.
This issue has been there for a long time, and you have been running
essentially as PAT disabled in the past. The commit in question simply
detected this issue.
Thanks,
-Toshi
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