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

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Message-ID: <20140225162611.GA31902@redhat.com>
Date:	Tue, 25 Feb 2014 11:26:11 -0500
From:	Dave Jones <davej@...hat.com>
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, Feb 25, 2014 at 02:45:57AM -0800, H. Peter Anvin 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.

About six years ago, we almost went down this same path for Fedora,
and I'm fairly sure the only reason we backed off and decided to not
pursue it was that we found some Pentium M's where it just didn't work.

	Dave

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