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SubjectRe: [PATCH] x86: set Pentium M as PAE capable
On 02/25/2014 08:26 AM, Dave Jones wrote:
> 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.
>

OK, that *definitely* means that if we're doing this at all we're doing
it via an explicit opt-in on the command line, and tainting the kernel
in the process.

I don't know if anyone (Chris?) is interested enough in the problem to
do such a patch, though. I know I'm not too interested in spending a
bunch of time on.

-hpa



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