linux-kernel - Re: [PATCH] x86: set Pentium M as PAE capable

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Message-ID: <20140225120743.6ed967ed@alan.etchedpixels.co.uk>
Date:	Tue, 25 Feb 2014 12:07:43 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Chris Bainbridge <chris.bainbridge@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: set Pentium M as PAE capable

On Tue, 25 Feb 2014 02:45:57 -0800
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 02/24/2014 10:01 PM, Chris Bainbridge wrote:
> > Pentium M is PAE capable but does not indicate so in the CPUID response.
> > This is an issue now that some distributions are no longer shipping
> > non-PAE kernels (those distributions no longer boot on Pentium M). This
> > small patch fixes the issue by forcing the PAE capability on Pentium M.
> >
> > For more discussion see https://bugs.launchpad.net/baltix/+bug/930447
> >
> 
> 1. This patch doesn't match the discussion in the link.
> 2. You would have to also enable this in the cpu testing code in
>     arch/x86/boot.
> 3. At the very least we need to print a serious warning that the CPU
>     is being run outside its specifications.  I have no personal
>     information about why this CPUID bit was disabled, but it could be
>     that it was discovered in testing that it didn't work correctly in
>     all circumstances (e.g. high temperature.)  This is very much "use
>     at your own risk..."; you could get data corruption or even
>     hardware damage.

A hang with an extended version of the no PAE message that warns you
and then says "Boot with the option "forcepae" to bypass this check",
would, IMHO, be a bit wiser, unless someone can actually dig out the
reason it does not advertise the flag.

Alan
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