linux-kernel - Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately?

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Date:	Mon, 4 Jun 2007 08:48:37 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Jesse Barnes" <jbarnes@...tuousgeek.org>
Cc:	"Matt Keenan" <tank.en.mate@...il.com>,
	"Justin Piszcz" <jpiszcz@...idpixels.com>,
	"Andi Kleen" <andi@...stfloor.org>,
	"Venki Pallipadi" <venkatesh.pallipadi@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately?

On 6/4/07, Jesse Barnes <jbarnes@...tuousgeek.org> wrote:
> On Sunday, June 3, 2007 2:15:06 Matt Keenan wrote:
> > Justin Piszcz wrote:
> > > On Sat, 2 Jun 2007, Andi Kleen wrote:
> > >>> I feel, having a silent/transparent workaround is not a good idea.
> > >>> With that
> > >>
> > >> If enough RAM is chopped off users will notice. They tend to complain
> > >> when they miss RAM.  I don't like panic very much because for many
> > >> users it will be a show stopper (even when they are not blessed
> > >> with "quiet" boots like some distributions do)
> > >>
> > >> The message in dmesg could be also emphasized a bit with a little
> > >> ASCII art (but no <blink> tag in there)
> > >>
> > >> The problem I'm more worried about is if the system will be really
> > >> stable --- could it be that the memory controller is still
> > >> misconfigured and cause other stability issues? (we've had such
> > >> cases in the past). Also I'm not sure we can handle the case of
> > >> the MTRR wrong not at the end of memory but at the hole sanely.
> > >>
> > >> -Andi
> > >
> > > So far I have been booting with mem=8832M and have run stress/loaded
> > > the memory subsystem pretty good; what other tests should I run?
> > >
> > > It'd be nice if we could pose some sort of solution/warning for the
> > > future so other people do not have to experience the same problems.
> > >
> > > What are the next steps?
> >
> > Wouldn't it be possible for the e820/MTRR set up code detect the problem
> > and suggest a mem=xxxx that would fix the problem (while also
> > complaining that the BIOS is broken)?
>
> Yes, that should be fairly easy, though as Andi points out, if there are holes
> in the MTRR setup, things get a little trickier (I had an earlier patch to
> deal with this, but ended up with too many early boot issues).
>
> Maybe what Venki suggested would be best:  just detect the condition and
> panic, with a string telling the user to use mem=xxx (we can figure that out)
> and/or upgrade their BIOS.

Ick. Systems that used to boot fine would then panic on a kernel
upgrade. That's rather rude for a condition that's merely an
optimization (using all memory), rather than one of correctness. A
panic seems entirely inappropriate.

Ray
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